AU737397B2 - Lossless active clamp for secondary circuits - Google Patents

Description

1

AUSTRALIA

Patents Act 1990 COMPLETE

SPECIFICATION

STANDARD

PATENT

LOSSLESS ACTIVE CLAMP

FOR

SECONDARY

CIRCUITS

The following statement is a full description of this invention, including the best method of performing it known to me: 1 LOSSLESS ACTIVE CLAMP FOR SECONDARY

CIRCUITS

FIELD OF INVENTION The present invention relates to the field of power converters in general, the clamping of voltages in a secondary circuit, and the substantially lossless recycling of the captured energy in particular. In one form, the present invention provides a substantially lossless active voltage clamp in the secondary circuit of an isolated power converter.

BACKGROUND

In the field of isolated power converters, the reverse recovery of secondary diodes results in a large current spike flowing in the transformer and the subsequent storing of energy in the transformer leakage inductance. The energy stored in the leakage inductance will cause ringing on diode voltage waveforms with large overshoots, necessitating the use of high voltage rated diodes, and causing heating of the transformer as the ringing is primarily damped out by transformer resistance. In medium to higher power converters the loss is substantial and limits the operating frequency making the use of relatively large transformers necessary.

The prior art is explained with reference to the basic forward converter shown in Figure 1.

The basic forward converter consists of an input voltage Vci impressed on capacitor C1; a transformer T1 with three windings: primary Lpri, clamp winding Lcamp and secondary Lsec, with a corresponding turns ratio of 1:1:N; a primary switch S1; a clamp diode D10; a secondary forward power conversion diode D1; a freewheel power conversion diode D2; and an output filter network L1 and C2 driving a load. Assuming S1, D10 and the transformer are ideal, with the associated total transformer leakage inductance being lumped in the secondary as inductor L2, forward conversion is made when primary switch S1 is tumrned on.

One method of controlling the voltage overshoot is to partially dissipate the energy by using a resistor-capacitor network across each of the secondary diodes. Figure 2 shows this type of prior art, being a dissipative snubber in an isolated forward converter. The voltage across power conversion diodes D1 and D2 are limited by voltage shaping capacitors C3 and C4, and dissipation elements R3 and R4. The resonant oscillation of voltage caused by the leakage inductance L2 and capacitors C3 and C4 is damped by resistors R3 and R4. The circuit is lossy, because most of the total energy stored in the leakage inductance, plus a fixed amount per cycle from the primary, is lost as heat in the damping resistors R3 and R4. The circuit typically limits the diode voltage to twice VLsec, 2 requiring power conversion diodes D1 and D2 to have high voltage ratings, but lower than the power conversion diodes in Figure 1.

Another prior art that limits the secondary diode voltage is the dissipative clamp shown in Figure 3. The voltages across power conversion diodes D1 and D2 are clamped by clamp diodes D3 and D4 when they swing to the levels of the voltages on capacitors C3 and C4.

Energy from the leakage inductance charges capacitor C3 when D3 is on, and C4 when D4 is on. Resistors R3 and R4 continuously discharge capacitors 03 and C4 respectively, controlling the capacitor voltages. The dissipation in resistors R3 and R4 is equal to the sum of the energy stored in the leakage inductance L2 each cycle plus a constant amount due to the minimum capacitor voltage equal to VLsc. The action of D3, C3 and R3 is to clamp the voltage across power conversion diode D1 to a level that depends on the energy stored in the leakage inductance during the tumrn off of D1. Similarly, the action of D4, C4 and R4 clamp the voltage across power conversion diode D2 during the tumrn off of D2. The clamps are dissipative because almost all of the energy transferred to the clamps is lost in either R3 or R4. The advantage of the clamps is that power conversion diodes with a lower voltage rating than those of Figure 1 or Figure 2 can be used for the power conversion diodes D1 and D2.

The prior art discussed above, limits the switching frequency of the power converter at medium to high power levels because the thermal limit of the dissipative elements is often reached. In addition, the overall efficiency of the power converter is not improved.

OBJECT OF INVENTION An object of the present invention is to overcome the problems associated with the above prior art.

Another object of the present invention is to provide a secondary power conversion diode voltage clamp that recovers the transformer secondary leakage energy in a substantially lossless manner for use with isolated power converters.

SUMMARY OF INVENTION The present invention is predicated on the principle of the clamping of power conversion diode voltages in a secondary circuit of an isolated power converter, and the substantially lossless recycling of the captured energy, thereby improving the efficiency of the power converter and permitting operation of the power converter at high switching frequencies.

The present invention, in one form as used in a forward converter, consists of a voltage clapomprising two additional clamp diodes and an additional clamp capacitor, and an

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energy recycling circuit for the recycling of energy stored in the clamp capacitor comprising an additional inductor, energy recycling switch and an energy recycling diode.

The operation of the clamp captures the energy associated with the power conversion diode reverse current flowing in the transformer secondary leakage inductance for all switching transitions of the converter. The energy is stored in the clamp capacitor, which clamps the reverse voltage of the power conversion diodes. The energy recycling switch is operated either during the on or off period of the primary switch, depending on the orientation of the energy recycling circuit, to reset the clamp capacitor voltage, resulting in a transfer of the corresponding energy either to the primary side for reuse, or to the load.

An increase in converter efficiency is obtained through both the recycling of the captured energy and by enabling the use of power conversion diodes with a lower voltage rating, which in general have lower conduction loss as compared to the high voltage diodes needed for the unclamped circuit.

One form of the present invention is now described with reference to the following drawings.

Figure 4 shows a forward converter with a substantially lossless active clamp for secondary circuits arranged for use during the off period of the primary switch; Figure 5 shows the timing waveforms for Figure 4; Figure 6 shows the equivalent circuit of the recycling circuit; Figure 7 shows a forward converter with a substantially lossless active clamp for secondary circuits arranged for use during the on period of the primary switch.

Figure 8 shows a forward converter with a substantially lossless active clamp for secondary circuits arranged for use during the on period of the primary switch with an alternative arrangement of the recycling circuit to allow maximum conduction time of the energy recycling switch.

Figure 9 shows a forward converter with a substantially lossless active clamp for secondary circuits arranged for use during the on period of the primary switch with an alternative arrangement of the recycling circuit using a magnetically coupled circuit to allow maximum conduction time of the energy recycling switch.

4 Referring to Figure 4 and Figure 5, the present invention clamps the voltage across power conversion diode D1 as it rises to the level of capacitor C5 voltage via D3 and power conversion diode D2. Similarly, the voltage across power conversion diode D2 is clamped as it rises to the level of capacitor C5 voltage via D4 and power conversion diode D1. In this way, during both switching transitions of the secondary the power conversion diode reverse voltages are clamped to a level to permit the use of power conversion diodes with lower voltage ratings, as occurs in the prior art circuit of Figure 3.

The recycling of the energy stored in capacitor C5 is achieved by tumrning on switch during the period when power conversion diode D2 is on. Inductor L5 is chosen to be typically much larger than the transformer leakage inductance and permits the tumrn on of under zero current conditions. Additionally, if clamp diode D3 is in conduction at the time switch S5 is turned on, switch S5 will be tumrned on under zero voltage condition, resulting in zero tumrn on loss. The absence of dissipation elements in the circuit makes the recycling of energy substantially lossless.

Full circuit operation of the present invention in a forward converter with a 1:1:1 tumrns ratio transformer is as follows, referring to Figure 4, Figure 5 and Figure 6: At time tO, the primary switch Si is tumrned on, forward biasing power conversion diode D1.

Magnetising current I(Mag) in the transformer increases linearly from zero.

Between tO and tl, the current in D1 increases at a rate determined by the transformer leakage inductance 12 and the voltage VLs. At the same time the current in power conversion diode D2 decreases at the same rate until at time tl the current reaches the reverse recovery current required to turn D2 off.

At tl, the power conversion diode D2 turns off and supports reverse voltage. The current in the leakage inductance L2 is equal to the output choke L1 current plus the diode reverse recovery current. The extra energy in L2 due to diode reverse recovery causes the reverse voltage on D2 to continue to increase to a level equal to the voltage on clamp capacitor C5. Clamp diode D4 becomes forward biased and provides a current path for the diode reverse recovery energy stored in L2 around the loop formed by D1, C5, D4 and the transformer secondary Lse.

Between tl and t2, the reverse recovery energy stored in L2 is transferred to clamp capacitor C5 as the current in 12 reduces to the load current at a rate proportional to the

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PFMJG

voltage difference between C5 and the ideal transformer secondary voltage VLSe. At the end of this period, current in L2 falls to the load current and clamp diode D4 turns off, releasing the voltage across the power conversion diode D2 to fall to the ideal transformer secondary voltage VLsec.

During period t2 to t3, primary switch S1 is delivering energy to the load via power conversion diode D1 and output choke L1. Voltage on clamp capacitor C5 remains unchanged from the peak value obtained at time t2.

At time t3, primary switch S1 is tumrned off. The voltage on the primary winding Lp, swings positive, due to the magnetising current I(Mag) and load current flowing in L2, until clamp winding Lc.p, and clamp diode D10 operate to clamp the primary switch voltage VLpn to twice the voltage on C1. Magnetising current I(Mag) then begins to decrease linearly as the magnetising energy is transferred back to primary storage capacitor C1 via D10 and Lcgap. The secondary terminal voltage VsEc similarly collapses, forward biasing power conversion diode D2.

From t3 to t4, the current in power conversion diode D2 increases at a rate dependent on the leakage inductance L2 and voltage V.Lsec Current in power conversion diode D1 decreases at the same rate until the reverse recovery current of D1 is reached at t4.

At t4, power conversion diode D1 turns off and supports reverse voltage. The current in the leakage inductance L2 is equal to the diode reverse recovery current. This current causes the reverse voltage on D1 to continue to increase to a level equal to the voltage on clamp capacitor C5. Clamp diode D3 becomes forward biased and provides a current path for the excess energy stored in L2 due to diode reverse recovery.

Between t4 and t5, the loop formed by D2, C5, D3 and the secondary winding Lm has the effect of transferring some of the energy in L2 to the primary storage capacitor Ci via the clamp winding Ldp and clamp diode D10. The rest of the energy is transferred to clamp capacitor C5 as the current in L2 reduces to zero at a rate proportional to the voltage difference between C5 and the ideal transformer secondary voltage VLsc (also equal to the reflected primary capacitor voltage Vcl.). At the end of this period, clamp diode D3 turns off, releasing the voltage across the power conversion diode to fall to the ideal secondary voltage VLsec.

6 During period t5 to t6, power conversion diode D2 is conducting the full current in output choke LI. Voltage on clamp capacitor C5 remains unchanged from the peak value obtained at time t5. For operation of the energy recycling circuit under zero voltage, the duration of period t5 to t6 is zero, since energy recycling switch S5 can be tumrned on in the period t4 to t5 with no effect on the clamp operation.

At time t6, the energy recycling circuit is activated by tumrning on energy recycling switch Since inductor L5 is carrying zero current, the tumrn on of S5 is under zero current conditions. With S5 on, the voltage difference between the ideal secondary winding of the transformer VLsec (=Vcl' in Figure 6) and capacitor C5 is applied across inductor L5 and leakage inductance L2 via the loop formed by C5, conducting power conversion diode D2 and the transformer secondary This is illustrated by the equivalent of the energy recycling circuit shown in Figure 6, where Ls is replaced by the transferred primary capacitor C1'. The effect of the increase in current in L5 12 is a net decrease in the current flowing in power conversion diode D2.

Between t6 and t7, the current increases in L5 L2, which for a moderate value of capacitance C5, can be assumed to be approximately linear as the voltage on C5 will not change by a significant amount over the full switching cycle. A portion of the energy in is transferred to the primary circuit energy storage capacitor Cl via primary clamp winding Lcamp and diode D10, adding to the magnetising current already flowing in the winding.

Another portion is transferred to L5 1L2. Time between t6 and t7 can be determined either by a controlled on time for S5 or by the current in S5 reaching a predetermined value.

At time t7, S5 is tumrned off. Current continues to flow in L5 L2, tumrning diode D6 on.

This impresses a negative voltage equal to the ideal transformer secondary voltage VLGc across inductor combination L5 and the leakage L2, reducing the current at a rate equal to voltage VLs divided by the inductance of L5 L2.

Between t7 and t8, all of the energy in L5 and L2 is returned to the primary via the loop formed by diode D6, the power conversion diode D2, the transformer secondary Lsec and series inductors L5 and L2. The primary clamp winding Lc,,p and clamp diode transfer the energy to energy storage capacitor C1. The equivalent of this loop is shown in the circuit of Figure 6, where Lsec is replaced by the transferred primary capacitor C1'.

When the current in L5 reaches zero, D6 tumrns off, disconnecting the recycling circuit.

Powerconverslon diode D2 again conducts the full current of output choke L1. Switch 7'OFR 7 must be turned off early enough so that time t8 is reached before the transformer magnetising current falls to zero to provide a coupling path for the energy to the primary circuit. The total time from t3 to t8 is equal in duration to the on time of primary switch S1.

Hence, the maximum energy recovery cycle duration is limited to the on time of primary switch S1 and is therefore a function of load demand.

Between t8 and t9, the magnetising current continues to flow in the clamp winding Lcamp and clamp diode D10 until it reaches zero at t9. The power conversion diode D2 is still in conduction over this period due to the continuous current flowing in output choke L1, since the converter is operating in continuous conduction mode.

At t9, the transformer core is reset and the primary and secondary voltages collapse to zero. Power conversion diode D2 continues conducting output choke L1 current until the next cycle begins at time tl 0.

The amount of energy, El, to be recycled by the present invention is a function of the magnitude of the diode reverse recovery current, the load currentflowing in the output choke L1 at the time forward conversion begins, ILI,), the leakage inductance L2, and is given by the equation: E, (1) This energy can be recycled in a short period of time if L5 L2 is small, requiring a high peak current to flow in switch S5 over a short on time. A more beneficial approach is to use a larger value of L5 and a longer conduction time of switch S5, so that for the same peak energy stored in L5 L2, the peak current in switch S5 is significantly smaller.

Referring to Figure 7, the same lossless active clamp for secondary circuits of Figure 4, is rearranged to permit operation of the energy recycling circuit during the period when the primary switch is on. This is achieved by connecting inductor L5 to the cathode of clamp diode 04 instead of to the cathode of clamp diode D3. The operation of the clamp components D3, 04 and C5 are the same as previously described and the operation of the energy recycling circuit is similar to that previously described except that the energy is delivered to the load instead of being returned to the primary energy storage capacitor Cl.

The operation of the energy recycling circuit supplies a direct portion of the load current around the loop inductor L5, switch S5, clamp capacitor C5, output choke L1 and the load during the on time of S5. At the end of the conduction period of switch S5, some of the

A

load current flows via inductor L5, diode D6, output choke L1 and the load. The net effect is a reduction of the load current supplied from the primary via power conversion diode D1.

The limitation of this arrangement is the same as for the arrangement of Figure 4, that the period available to complete the operation of the energy recovery cycle is dependent on the on time of the primary switch S1, which varies with converter load demand. The minimum time period available for the arrangement of Figure 4 and Figure 7 occurs when the output of the converter is under short circuit conditions. Under these conditions, the energy recycling circuit may not always be able to reset the clamp capacitor 05 and a dissipative clamp Z1 (zener) is required to prevent breakdown of power conversion diodes D1 and D2.

An altemrnative arrangement, as shown in Figure 8, is an improvement of Figure 7 which maximises the conduction time of energy recycling switch S5 by providing an alternative reset voltage for inductor L5. The diode D6 is connected to output capacitor C2. With this arrangement and tumrning on switch S5 while clamp diode 04 is in conduction, for zero voltage tumrn on, the conduction time of S5 can be the same duration as the primary switch S1. This maximises the energy recovered from clamp capacitor 05, and the current in inductor L5 can be reset independently of the secondary winding voltage Vse when S5 is turned off, delivering all the energy stored in L5 to the load.

A further advantage of the arrangement of Figure 8 is that the control of switch S5 can be simplified to coincide with the conduction period of power conversion diode D1. This is because the switch S5 can be tumrned on as soon as the clamp diode D4 conducts and is required to be tumrned off at the same time as primary switch S1.

Another altemrnative arrangement, as shown in Figure 9, provides the same advantages as the arrangement of Figure 8, but exhibits additional flexibility for the resetting of the magnetic flux in L5 via a coupled winding for that purpose. The reset winding is connected to output capacitor C2 via diode D6 and delivers all the energy stored in L5 to the load when switch S5 is tumrned off. This arrangement permits the connection of the output choke L1 in the negative output line of the power converter, while still retaining all the advantages of the arrangement of Figure 8.

The present invention can be applied to the secondary circuits of other converter /-A'topologies with minimal rearrangement of the clamp and energy recycling circuit f?4\ 7X

-A/

9 components. The converter topologies to which the present invention can be arranged for includes, but is not limited to: flyback, half-bridge, full bridge, isolated sepic, isolated zeta and isolated cuk; and where secondary rectifiers are arranged as bridge rectified, center tapped, and current doubler circuits. The present invention also can be applied as an additional circuit to resonant transition variations of the above mentioned topologies.

Claims (4)

1. In an isolated power converter including, in a loop, a transformer secondary winding containing leakage inductance, a power conversion diode or diodes which exhibit reverse recovery, an energy storage filter and a load; the improvement comprising: a voltage clamp for each power conversion diode comprising a single clamp capacitor element and one or more clamp diodes, for transferring energy stored in the secondary winding leakage inductance due to reverse recovery of the power conversion diode or diodes to the clamp capacitor, thus limiting the voltage across each of the power conversion diodes; and an energy recycling circuit including a magnetic energy storage element, an energy recycling switch means controlled by a variable pulsewidth, synchronised with one or more of the power conversion switching transitions, where the peak current carried by the switch means is substantially less than that carried by the power conversion diodes, and an energy recycling diode; for transferring energy stored in the clamp capacitor to either the load or the primary circuit.

An improvement as claimed in 1, where the switch means is controlled by a constant pulse width synchronised .with one or more of the power conversion switching transitions.

3. An improvement as claimed in 1, where the switch means is controlled by a signal with the same duty cycle as and synchronised with, one of the power conversion diodes, thereby making a simple control.

4. An improvement as claimed in 1 to 4, substantially as herein described with reference to the figures 4 to 9 of the accompanying drawings. An improvement as claimed in 1 to 4, where the essential elements of the improvement as claimed in 1 are rearranged to permit operation of the voltage clamp and energy recycling circuit in association with other isolated power conversion topologies, including but not limited to: flyback, half-bridge, full bridge, isolated sepic, isolated zeta and isolated cuk; and where secondary rectifiers are arranged as bridge rectified, center tapped, and current doubler circuits. Sheehy Dekter 16 MAY 2001