GB2371929A

GB2371929A - DC-DC converter

Info

Publication number: GB2371929A
Application number: GB0126352A
Authority: GB
Inventors: John Stephens
Original assignee: Advanced Power Conversion Ltd
Current assignee: Advanced Power Conversion Ltd
Priority date: 2001-02-02
Filing date: 2001-11-02
Publication date: 2002-08-07
Also published as: AU2002234816A1; GB0126352D0; WO2002061925A2; WO2002061925A3; GB0102674D0

Abstract

A DC-DC converter comprises a primary circuit fed by a DC power source. The primary circuit comprises a transformer primary winding X<SB>p</SB> having a first end coupled to one side of the DC power source through a capacitor C1 and inductor L1P, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches Rs11,Rs12 connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding X<SB>s</SB> and rectifying means Rs3,Rs4 for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately. The control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means. In an alternative embodiment the control circuitry operates at least one of the switches when there is no voltage across it. In a further embodiment the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for. The switching means may be MOSFETs.

Description

A Converter The present invention relates to a converter, typically for a power supply for supplying a continuous output current, from a continuous input current, with particular applications, amongst others, as power supplies for examples in automotive or telecoms applications.

Transformers used in electrical and electronic applications for 'transforming'an input voltage to a higher or lower voltage (and often referred to as"Buck"and"Boost"converters respectively) are well known to persons skilled in the art. A problem with known transformers is to provide assemblies which operate with both continuous input and output currents. This is possible with a series of boost and buck converters, but a simple cascade of these two has an increased component count and is additionally complex to drive the devices.

A known DC-DC converter is described in US 5 886 882 (Rodolpho), which features primary and secondary transformer windings, together with two pairs of primary and secondary choke windings, wound upon a three limbed core. A switching circuit is coupled between the first primary choke and the primary transformer winding, and a similar switching circuit is coupled between the first secondary choke and the primary transformer winding. Each switching circuit comprises a capacitor, diode and a MOSFET. The switching circuits are switched on and off in a cyclic manner (the MOSFETs being driven by two interleaved square pulse trains) to provide a continuous output current from a continuous input current in a push-pull manner.

It is an object of the present invention to provide a DC to DC converter and method of driving it to efficiently produce a continuous output current for a continuous input current.

According to the present invention there is provided a DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately; characterised in that the control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means.

Preferably, the second end of the transformer primary winding is connected to the DC power source through a second capacitor and second inductor, the second capacitor being coupled to the second side of the DC power source through a second switching means comprising a second pair of switches connected to the inductor side and transformer primary winding side of the capacitor; Preferably, there is provided a delay between one capacitor discharging through the primary coil, and the other capacitor discharging through the primary coil.

According to another aspect of the invention, there is provided a DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having each end coupled to the same side of the DC power source through a respective capacitor and inductor, each capacitor being coupled to the second side of the DC power source through respective switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of each switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of each capacitor alternately, such that each capacitor discharges through the primary winding alternately; characterised in that the control circuitry operates the at least one of the switches when there is substantially no voltage across it.

According to another aspect of the invention, there is provided a DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having each end coupled to the same side of the DC power source through a respective capacitor and inductor, each capacitor being coupled to the second side of the DC power source through respective switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of each switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of each capacitor alternately, such that each capacitor discharges through the primary winding alternately; characterised in that for one or both switch means, the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.

A converter according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram of a converter; Figure 2 shows the current flow during the basic phases of a cycle in the converter; Figure 3 shows the voltages and currents across particular components due to the basic phases; Figure 4 shows the switching timings of the converter; Figures 5 to 9 shows other embodiments of the converter; Figure 10 shows alternative switching timings of the converter; and Figure 11 shows a further embodiment of the converter; Referring to Figure 1, the converter comprises an input, an output, and a transformer assembly having a ferrite core. The core is formed from two'E'shaped core pieces, each core piece having three limbs. The core pieces are placed together to form three limbs. Primary and secondary transformer windings xp and x, are provided on the centre limb of the ferrite core. Windings Lip and Lis are provided on an outer limb to form primary and secondary chokes, and windings L2p and Lzs on the other outer limb form a second pair or primary and secondary chokes. Other core shapes, such as'I'shaped pieces, may be used.

A capacitor C, is provided between the primary transformer winding xp and the first primary choke winding L, p. Similarly, a capacitor C2 is provided between the primary transformer winding xp and the second primary choke winding L2p. Two switches RS, ; and RS are coupled between the capacitor C, and the first primary choke winding Lip, and capacitor C2 and the second primary choke winding L2p respectively.

Similarly, two switches RS and RS22 are coupled between the capacitor CI and the primary transformer winding xp, and capacitor C2 and the primary transformer winding Xp. The switches RS, ; and Rus 2 and the capacitor C, form a first switching circuit for the first primary choke winding Lip, and the switches RS2, and RS22 and the capacitor C2 form a second switching circuit for the second primary choke winding L2p An input voltage is applied to the first primary choke winding L, p and the first switching circuit, and to the second primary choke winding L2p and the second switching circuit.

In the secondary circuit, the secondary transformer winding Xs is connected between the first secondary choke winding Lis and the first secondary choke winding L2,. Two switches RS3 and RS4 are coupled to points between the secondary transformer winding Xs and the first secondary choke winding Lis, and between the secondary transformer winding Xs and the second secondary choke winding L2s. A capacitor Co is coupled across the output for smoothing.

Referring to figure 2, the basic cycle of the circuit has four separate phases. In this figure, the first primary choke is labelled Lxfmr1, the second

primary choke is labelled L, the primary transistor winding is labelled Lxfmr3 the secondary transistor winding is labelled L, first secondary , fmr33, choke is labelled Lrlh the second secondary choke is labelled Lr22-It will also be noticed that for the purposes of explaining the basic mode of operation, the pair of switches RS ; ; and RS, has been illustrated as a single switch having two contact positions RS11 and RSi2. Switch pair RS21 and RS22 have been simplified in a corresponding manner. A capacitor C, is added across the voltage source (here a battery) to smooth the input.

In an initial state (say before to), switches RS12 and RS22 are closed in the primary circuit, while RS11 and RS21 are open. Capacitor C, is

charged through the first primary choke winding L, p, and similarly capacitor C2 is charged through the second primary choke winding L2p at the input voltage. Capacitors C1 and C2 are sufficiently large to smooth the ripple voltage caused by switching, and the choke windings L1p and L2p are sufficiently large to smooth the ripple current.

Figure 3 shows the currents and/or voltages across various of the components in the circuit through two switching cycles. Figure 3 shows respectively the current across the switches RS11, RS12, RS21, and RS22, the voltage across the switches RSj,, RS12, RS21, and RS22, the current across

the first and second primary choke windings LIp and LIp, the current through the capacitors C, and C2, the voltage across the primary transformer winding xp, the magnetic current across the primary transformer winding xp, the total current across the primary transformer winding xp, the voltage across the secondary transformer winding Xs, the current through the diodes D, and D2, (which are substantially equivalent to the switches RS3 and RS4 of figure 1) the current across the first and second secondary choke windings L, s and Lis, and the current through the smoothing capacitor Co.

Referring back to Figure 2, at a time to, switch RS, ; is closed and RS12 is opened. The voltage across C, is applied upon the primary transformer winding xp, the secondary voltage at xs reverse biases D, and

causes the current to increase through x,. The current in D2 increases to the sum of both the currents flowing in Lfn and L. The output voltage is dependent upon the turn ratio of the primary and secondary transformer windings xp and xs, the secondary choke arrangement halving the output voltage, half the current flowing through each inductor.

At a time t1, switch RS 1 is opened and RS, is closed. The primary

transformer winding xp is clamped by RS22 and RS12 to 0 volts, the stored p'2and RS, 2 to 0 volts, the stored energy in the primary transformer winding xp circulating as current. The secondary transformer winding xs is clamped by the primary transformer winding. Energy stored in the first secondary choke winding circulates as current through D,. Current through the first primary choke L, p decreases and capacitor C, recharges. The current through Ls decreases.

At a time t2, switch RS22 is opened and RS21 is closed. The charge on capacitor C2 discharges through the primary transformer winding xp, taking the lower connection negative and causing a current to flow through D, and xs. As previously, the output voltage is dependent upon the turn ratio of the transformer windings.

At a time t3, switch RS21 is opened and RS22 is closed. The primary transformer winding xp is clamped by RS and RS ; 2 to 0 volts, the stored energy in the primary transformer winding xp circulating as current. The secondary transformer winding Xs is clamped by the primary transformer winding. Energy stored in the second secondary choke winding circulates as current through D2. This phase resets the transformer. At t4, the circuit switches as described from to, and the cycle is repeated indefinitely.

It can be shown that the output voltage, VOX Vo =N,/Np. V,. D/ (l-D) where V, is the input voltage,. N/Np is the transformer turn ratio, and D is the duty cycle of the switching circuits.

The switches are operated by a control circuit (not here shown). A typical switching cycle would be over a 5 s period, as shown in Figure 4.

Initially, only switch RS12 is closed, whilst all the other switches are open.

After 0. 1 s, switch RS22 is closed. After 0. 7s from the beginning of the cycle switch RS12 is opened. At 0. 8Rs from the beginning of the cycle, RS11 is closed. 2. 5 s through the cycle, RS11 is opened. At 2. 6 s from the beginning of the cycle, switch RS12 is closed. At 3. 2us from the beginning of the cycle, switch RS22 is opened, and at 3. 3/lus from the beginning of the

cycle, switch RS2, is closed. Switch Ris2, is opened 5us from the beginning of the cycle, which marks the start of a new cycle.

It will also be seen that during this time, diodes D, and D2 follow the timings of RS12 and RS22 respectively. Switches, such as MOSFETs could though be used instead or diodes.

When each switching circuit is switched, it will be seen that there is a delay between the first switch opening and the second switch closing, (i. e. between RS ; 2 opening and RS22 closing, between RS22 closing and RS, 2 closing, between RS22 opening and RS21 closing, and between RS21 opening and RS22 closing). Under this timing sequence, the delay is 0. 11 s, but the optimum delay is dependant upon the components in the circuit, throughput power, supply voltage, operational switching frequency and other effects.

Typical delays range from 50ns to 100ns.

The ferrite core indicated in figure 1 as a dotted line, and as previously mentioned, is formed from two'E'or'I'shaped core pieces.

The ferrite cores need not be in this integral form however. Referring to

figure 5, the windings L ; p, L, L2p and L2s may be provided as discrete magnetic components (Windings xp and xs sharing a single magnetic core in this and every other embodiment).

Referring to Figure 6, the first and second primary choke windings

LIP and L2p may be magnetically coupled to the transformer windings xp and Xg. The first and second secondary choke windings Lis and L2s are each magnetically discrete components. Conveniently, the first and second primary choke windings LIP and L2p are wound upon the outer limbs of the core, while the first and second secondary choke windings LI, and L2s are provided as discrete inductors.

Referring to Figure 7, the first and second secondary choke windings LIs and Lzs are magnetically coupled to the transformer windings xp and Xs, the first and second primary choke windings L, p and L2p being magnetically discrete components. In a similar manner to the previous embodiment the first and second secondary choke windings LI, and Lzs are conveniently wound upon the outer limbs of the core, while the first and second primary choke windings L1p and Lop are provided as discrete inductors.

In an alternative circuit, shown in figure 8, the first primary choke

winding LIP is magnetically coupled to the first secondary choke winding L in a discrete magnetic component, and the second primary choke winding L2p is likewise magnetically coupled to the second secondary choke winding Lzs.

It will be realised of course that different inductors may be coupled by different degrees. Rather than all the inductors (with the exception of the transformed inductors, which are always substantially tightly coupled) being discrete components with no coupling between them, some inductors may be loosely coupled.

A fully or partially integrated form of circuitry, where the primary and secondary choke windings are coupled, allows the input and output ripple currents to be steered so that the net output is relatively free from ripple.

The switching of a switching circuit associated with a particular winding applies an AC waveform, via the decoupling capacitor, to a any second or further winding which is magnetically coupled to the first. By

applying an AC waveform to the second winding matching the switching waveform of the first winding, the current flow into the first winding, caused by the switching action, can be halved. Also, by changing the turns ratio between the first and second windings, it becomes possible to further reduce the ripple current in the first winding, to the extent that the switching frequency ripple current can be reduce close to zero. This technique has the effect of apparently increasing the inductance of the first winding to a value significantly greater than the actual electronic value.

Referring to Figure 9, capacitors Crsll and Crs21 may be provided in parallel with switches RS,, and RS2,. The primary transformer winding is in reality not completely coupled to the secondary winding, but includes a component of pure inductor which is represented here as LZVRT.

The capacitances Crsll sn and C are coupled with the primary transformer winding's reactance to establish a resonant circuit such that the switching is effected at zero volts.

When RS) ; is closed, the charge accumulated on Crs II discharges through RS11, and Crs21 similarly discharges on RS's closing. In this manner, a waveform is obtained that counteracts the effect of the parasitic inductance LZVRT and reduces the losses otherwise attributable to it.

Rs21 and Rs22 are, by circuit operation, switched-ON with zero volt across them. On opening, the current flow is through the capacitors due to the capacitors charging-up (CV=IT). The capacitance, rather than being provided as a discrete component, may be an integral parasitic feature of the MOSFET (Coss). If it is external to the MOSFET i. e. additional capacitors as shown, then there are greatly reduced turn-OFF losses in RS, and RS21.

The inductance Lzvrt and the capacitors Crs11 and Crs21 form a resonant tank swinging the voltage across the switch to zero at which point the MOSFET, Ru 11 or Rs21 as the case may be, are switched-ON.

By switching the MOSFETs at the correct time and utilising ZVRT (Zero Volt Resonant Transition) switching noise (and the losses it causes) of RS11 and RS21 can be reduced. It is dependant upon the rate of change of voltage across the MOSFETs, this being controlled by the capacitors Crs11 and Crs21.

Other switching regimes may be followed. Such a further switching regime is shown in Figure 10. As is the previous example, the switching cycle is over a 5 s period. Initially, only switches D, and D, (referring also to the circuit in Figure 2) are closed, all the other switches being open.

After 0. llls from the beginning of the cycle, switches RS22 is closed. After

O. Sjis from the beginning of the cycle switch RS I, is closed whilst Dl is opened. At 2. 5 ; j. s from the beginning of the cycle, switches RS11 and RS22 are opened whilst D, is closed. 2. 611S through the cycle, switch RS2, is closed. At 3. 3 s from the beginning of the cycle, switch RS21 is closed and

D2 is opened. At the end of the cycle, Le. 511s from the beginning of the cycle, switches RS12 and RS21 are opened and D2 is closed. The cycle then repeats.

In this timing regime, it can be seen that the switches RS, I and RS22 are kept open for a longer period than in the previous timing regime. The switches therefore are not switched at zero volts, and cannot be switched in a resonant manner to reduce the inductive losses of the transformer winding. When in the open position however, the switches conduct no current and therefore will not dissipate energy, so the circuit is made more efficient.

The primary circuit may drive two or more secondary circuits.

Figure 11 shows two secondary circuits (the second secondary circuit having similar components as the secondary circuit already described) magnetically coupled to the primary circuit. The windings and capacitance's may of course be different, so that the two secondary circuits give different output voltages.

The switches described here have been MOSFETs, but other switching devices could substituted. Conveniently, RS),, RS12, D, and D2 could be n-channel MOSFETs, while RS2\ and RS22 are p-channel

MOSFETs. Alternatively, a p-channel MOSFET with a Schottky diode coupled across it could be used for the switches RS21 and RS22. RS2, and RS22 could also be n-channel MOSFETs with a Schottky diode coupled across it. Diodes D, and D2 could conveniently be Schottky diodes. Other suitable transistors or other switching devices will be apparent to one skilled in the art.

The principles disclosed herein could equally be applied to other DC-DC converter topographies, such as circuits having only one discharging capacitor in the primary circuit, and a correspondingly simplified secondary circuit.

Claims

Claim 1. A DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately; characterised in that the control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means.
2. A converter according to claim l, wherein the second end of the transformer primary winding is connected to the DC power source through a second capacitor and second inductor, the second capacitor being coupled to the second side of the DC power source through a second switching means comprising a second pair of switches connected to the inductor side and transformer primary winding side of the capacitor;
3. A converter according to previous claim 2 characterised in that there is provided a delay between one capacitor discharging through the primary coil, and the other capacitor discharging through the primary coil.
4. A converter according to claim 3 characterised in that period of the delay is approximately the same as the period over which one of the capacitors is discharged.
5. A converter according to any previous claim characterised in that the control circuitry operates at least one of the switches when there is substantially no voltage across it.
6. A converter according to any previous claim characterised in that for one or both switch means, the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.
7. A converter according to any previous claim characterised in that the control circuitry operates the switches such that the switch coupled when the voltage across it is substantially zero.
8. A converter according to any previous claim characterised in that the transient delay is approximately 1 J. s.
9. A DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together ; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately; characterised in that the control circuitry operates the at least one of the switches when there is substantially no voltage across it.
10. A DC-DC converter comprising: a primary circuit fed by a DC power source; the primary circuit comprising a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together ; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately; characterised in that for one or both switch means, the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.
11. A primary circuit according to any previous claim.
12. A converter as herein described and illustrated.
13. Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).