WO2002061928A2

WO2002061928A2 - A converter

Info

Publication number: WO2002061928A2
Application number: PCT/IB2002/000390
Authority: WO
Inventors: John Stephens
Original assignee: Advanced Power Conversion Ltd
Priority date: 2001-02-02
Filing date: 2002-02-01
Publication date: 2002-08-08
Also published as: WO2002061928A3; GB2371928A; GB0126316D0; AU2002228291A1; GB0102675D0

Abstract

A DC-DC converter comprises a transformer, the primary coil of which is connected to a primary circuit, and the secondary coil of which is connected to a secondary circuit. The primary circuit, includes at least a first primary capacitor and a first primary choke winding, the primary capacitor being charged and discharged in an cyclic manner, such that an AC current flows through the primary coil when a DC source is connected to the primary circuit. The secondary circuit, includes at least a first secondary choke winding, and rectification means, such that an AC current induced in the secondary coil is substantially converted to a DC output of the secondary circuit, characterised in that the first primary choke winding being connected in series with an first primary external inductor whilst not being substantially coupled to the first primary external inductor, and/or the first secondary choke winding being connected in series with a first secondary inductor whilst not being substantially coupled to the first secondary external inductor. The first primary choke winding and the first secondary choke winding may be inductively coupled.

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 transformer, the primary coil of which is connected to a primary circuit, and the secondary coil of which is connected to a secondary circuit,

the primary circuit, including at least a first primary capacitor and a first primary choke winding, the primary capacitor being charged and discharged in an cyclic manner, such that an AC current flows through the primary coil when a DC source is connected to the primary circuit,

the secondary circuit, including at least a first secondary choke winding, and rectification means, such that an AC current induced in the secondary coil is substantially converted to a DC output of the secondary circuit,

characterised in that the first primary choke winding being connected in series with an first primary external inductor whilst not being substantially coupled to the first primary external inductor, and/or

the first secondary choke winding being connected in series with a first secondary inductor whilst not being substantially coupled to the first secondary external inductor. Preferably the first primary choke winding and the first secondary choke winding are inductively coupled.

Preferably the primary circuit includes a second primary capacitor and a second primary choke winding, the second primary capacitor being charged and discharged in an cyclic manner alternately with the first primary choke,

and the secondary circuit including at least a second secondary choke winding.

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 and 6 show other embodiments of converters;

Figure 7 shows an alternative switching timing of the converter; and

Figure 8 shows another 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 Ε' shaped core pieces, each core piece having three limbs. The core pieces are placed together to form three limbs. Primary and secondary transformer windings x_p and x_s are provided on the centre limb of the ferrite core. Windings L_lp and L_ls are provided on an outer limb to form primary and secondary chokes, and windings L_2p and L_2s on the other outer limb form a second pair or primary and secondary chokes. Other core shapes, such as T shaped pieces, may be used.

A capacitor is provided between the primary transformer winding x_p and the first primary choke winding L_lp. Similarly, a capacitor C₂ is provided between the primary transformer winding x_p and the second primary choke winding L_2p. Two switches RS_π and RS₂₁ are coupled between the capacitor and the first primary choke winding L_lp, and capacitor C₂ and the second primary choke winding L_2p respectively. Similarly, two switches RS₁₂ and RS₂₂ are coupled between the capacitor and the primary transformer winding x_p, and capacitor C₂ and the primary transformer winding x_p. The switches RS_π and RS₁₂ and the capacitor Cj form a first switching circuit for the first primary choke winding L_lp, and the switches RS₂₁ and RS₂₂ and the capacitor C₂ form a second switching circuit for the second primary choke winding L_2p

An input voltage is applied to the first primary choke winding L_lp and the first switching circuit, and to the second primary choke winding L_2p and the second switching circuit. In the secondary circuit, the secondary transformer winding x_s is connected between the first secondary choke winding L_ls and the first secondary choke winding L_2s. Two switches RS₃ and RS₄ are coupled to points between the secondary transformer winding x_s and the first secondary choke winding L_ls, and between the secondary transformer winding x_s and the second secondary choke winding L_2s. A capacitor C₀ 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 L_xfoιrl, the second primary choke is labelled L_xfhιr2, the primary transistor winding is labelled L_Xfmr3> the secondary transistor winding is labelled L_xftnr33, first secondary choke is labelled L_xfinrll, the second secondary choke is labelled L_xfinr22. It will also be noticed that for the purposes of explaining the basic mode of operation, the pair of switches RS_π and RS₁₂, has been illustrated as a single switch having two contact positions RS_n and RS₁₂. Switch pair RS₂₁ and RS₂₂ have been simplified in a corresponding manner. A capacitor is added across the voltage source (here a battery) to smooth the input.

In an initial state (say before t₀), switches RS₁₂ and RS₂₂ are closed in the primary circuit, while RS_n and RS₂₁ are open. Capacitor Cj is charged through the first primary choke winding L_lp, and similarly capacitor C₂ is charged through the second primary choke winding L_2p at the input voltage. Capacitors C_x and C₂ are sufficiently large to smooth the ripple voltage caused by switching, and the choke windings L_lp and L_2p are sufficiently large to smooth the ripple current.

Also included are external inductors L_la and L_lb in series with L_lp, and L_2p respectively. When the inductors L_lp, L_ls, L_2p and L_2s are tightly coupled, a ripple in the input and output currents may result. Providing loosely coupled external inductors introduces an additional element which negates or 'steers' the unwanted ripple current, as further discussed below. As well as providing external inductors on both primary and secondary circuits, external inductors can also be provided solely on the primary circuit, or solely on the secondary circuit.

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 RS_π, RS₁₂, RS₂₁, and RS₂₂, the voltage across the switches RS_π, RS₁₂, RS_2l5 and RS₂₂, the current across the first and second primary choke windings L_lp and L_lp, the current through the capacitors and C₂, the voltage across the primary transformer winding x_p, the magnetic current across the primary transformer winding x_p, the total current across the primary transformer winding x_p, the voltage across the secondary transformer winding x_s, the current through the diodes D_r 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_ls and L_ls, and the current through the smoothing capacitor C₀.

Referring back to Figure 2, at a time t₀, switch RS_π is closed and RS₁₂ is opened. The voltage across Cj is applied upon the primary transformer winding x_p, the secondary voltage at x_s reverse biases Ω_x and causes the current to increase through x_s. The current in D₂ increases to the sum of both the currents flowing in L_xfinrll and L-. The output voltage is dependent upon the turn ratio of the primary and secondary transformer windings x_p and x_s, the secondary choke arrangement halving the output voltage, half the current flowing through each inductor. At a time t_ls switch RS_π is opened and RS₁₂ is closed. The primary transformer winding x_p is clamped by RS₂₂ and RS₁₂ to 0 volts, the stored energy in the primary transformer winding x_p circulating as current. The secondary transformer winding x_s 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_lp decreases and capacitor recharges. The current through L_s decreases.

At a time t₂, switch RS₂₂ is opened and RS₂₁ is closed. The charge on capacitor C₂ discharges through the primary transformer winding x_p, taking the lower connection negative and causing a current to flow through D_λ and x_s. 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 x_p is clamped by RS₂₂ and RS₁₂ to 0 volts, the stored energy in the primary transformer winding x_p circulating as current. The secondary transformer winding x_s 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 t₀, and the cycle is repeated indefinitely.

It can be shown that the output voltage, N₀,

V₀ = Ν_s/Ν_p . V_; . D/(l-D)

where N; is the input voltage,. Ν_s/Ν_p 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 5μs period, as shown in Figure 4. Initially, only switch RS₁₂ is closed, whilst all the other switches are open. After 0.1 μs, switch RS₂₂ is closed. After 0.7μs from the beginning of the cycle switch RS₁₂ is opened. At 0.8μs from the beginning of the cycle, RS_π is closed. 2.5μs through the cycle, RS_n is opened. At 2.6μs from the beginning of the cycle, switch RS₁₂ is closed. At 3.2μs from the beginning of the cycle, switch RS₂₂ is opened, and at 3.3μs from the beginning of the cycle, switch RS₂₁ is closed. Switch RS₂₁ is opened 5μs 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_t and D₂ follow the timings of RS₁₂ and RS₂ respectively. Switches, such as MOSFETs could though be used instead or diodes.

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 and/or adding external inductances, 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. The ferrite core indicated in figure 1 as a dotted line, and as previously mentioned, is formed from two 'E' or T shaped core pieces.

Referring to figure 5, four external inductors L_lp(ext), L_2p(ext), L_ls(ext), and L_2s(ext) are provided in series with L_lp, L_ls, L_2p and L_2s respectively. When the inductors L_lp, L_ls, L_2p and L_2s are tightly coupled, a ripple in the input and output currents may result. Providing loosely coupled external inductors introduces an additional element which negates or 'steers' the unwanted ripple current. As previously shown in figure 2, external inductors may be provided solely on the primary circuit (i.e. L_lp(ext) and

L_2p(ext) only). Equally, they may be provided solely on the secondary circuit (i.e. L_ls(ext) and L_2s(ext) only).

It will be realised of course that different inductors may be coupled by different degrees. Where inductors (other than the primary and secondary transformer windings) are magnetically coupled to a significant degree, external inductors loosely coupled or not substantially coupled may be introduced in series, so that the input and/or output ripple currents are steered to reduce the ripple currents.

The need for external inductors, and what value inductance should be used, will depend upon the magnetic properties of the circuit. In general, magnetically integrated circuits which are tightly coupled and have a low leakage inductance will benefit more than partially integrated or discrete circuits which will be loosely coupled and have a significant leakage inductance. If, for example, the transformer and choke windings are provided on a printed circuit board, the circuit will typically be tightly coupled (i.e. have a low leakage inductance), and the provision of external inductors (not here shown) will reduce the ripple current. The external inductors may be provided either in series with both primary choke windings, or in series with both secondary choke windings, or in series with both.

Referring to Figure 6, capacitors C_rsll and C_rs21 may be provided in parallel with switches RS_n and RS₂₁. 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 L_ZVRT.

The capacitances C_rsll and C_rs21 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_n is closed, the charge accumulated on C_rsll discharges through RS_π, and C_rs21 similarly discharges on RS₁₂'s closing. In this manner, a waveform is obtained that counteracts the effect of the parasitic inductance L_ZVRT 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 (C_oss). If it is external to the MOSFET i.e. additional capacitors as shown, then there are greatly reduced turn-OFF losses in RS_π and RS₂₁.

The inductance L_zvrt and the capacitors C_rsll and C_rs21 form a resonant tank swinging the voltage across the switch to zero at which point the MOSFET, Rsll 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 RS_π and RS₂₁ can be reduced. It is all dependant upon the rate of change of voltage across the MOSFETs, this being controlled by the capacitors Crsl 1 and Crs21. External chokes may be provided coupled either to the primary chokes, the secondary chokes, or to both.

Other switching regimes may be followed. Such a further switching regime is shown in Figure 7. As is the previous example, the switching cycle is over a 50μs period. Initially, only switches D_x and D₂ are closed, all the other switches being open. After 0.1 μs from the beginning of the cycle, switches RS₂₂ is closed. After 0.8μs from the beginning of the cycle switch RS_π is closed whilst D_t is opened. At 2.5μs from the beginning of the cycle, switches RS_π and RS₂₂ are opened whilst O₁ is closed. 2.6μs through the cycle, switch RS₂₁ is closed. At 3.3μs from the beginning of the cycle, switch RS₁₂ is closed and D₂ is opened. At the end of the cycle, i.e. 5μs from the beginning of the cycle, switch RS₁₂ is opened and RS₂₁ and D₂ are closed. The cycle then repeats.

In this timing regime, it can be seen that the switches RS_π 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 similar secondary circuits, as shown in Figure 8, each secondary circuit magnetically coupled to the primary circuit. External inductors may be fitted in series with the chokes of the primary circuit, and/or either or both the secondary circuits. 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_n, RS₁₂, D_λ and D₂ could be n-channel MOSFETs, while RS₂₁ and RS₂₂ are p-channel

MOSFETs. Alternatively, a p-channel MOSFET with a Shottky diode coupled across it could be used for the switches RS₂₁ and RS₂₂. Diodes Dj and D₂ could conveniently be Shottky 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

1 A DC-DC converter comprising:

the first secondary choke winding being connected in series with a first secondary inductor whilst not being substantially coupled to the first secondary external inductor.

2. A converter according to claim 1 characterised in that the first primary choke winding and the first secondary choke winding are inductively coupled.

3. A converter according to claim 1 characterised in that the primary circuit includes a second primary capacitor and a second primary choke winding, the second primary capacitor being charged and discharged in an cyclic manner alternately with the first primary choke,

and the secondary circuit including at least a second secondary choke winding.

4. A converter according to claim 3 characterised in that the second primary choke winding is inductively coupled to the first primary choke winding.

5. A converter according to either claim 3 or 4 characterised in that the second primary choke winding is connected in series with a second primary external inductor whilst not being substantially coupled to the second primary external inductor.

6. A converter according to any of claims 2 to 5 characterised in that the second secondary choke winding is connected in series with a second secondary external inductor whilst not being substantially coupled to the second secondary inductor.

7. A converter according to claim 6 characterised in that the second secondary choke is inductively coupled to the first secondary choke.

8. A converter according to either claim 6 or 7 characterised in that the second secondary choke is inductively coupled to the second primary choke.

9. A converter according to any of claims 2 to 8 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.

10. A primary circuit according to any previous claim, characterised in that the transformer coils and the choke windings are provided by a conductive paths deposited on a planar or laminate structure, and the external inductor or inductors are provided by discrete components.

11. A converter as herein described and illustrated.

12. Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).