# US 10,770,977 B2 · Systems and methods of operation for power converters having series-parallel mode active clamps

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- Number: US 10,770,977 B2
- Document ID: US10770977B2
- Status: Granted
- Kind: B2
- Application number: 16/366,611
- Filed: 2019-03-27
- Published: 2019-07-18
- Granted: 2020-09-08
- Earliest claimed: 2015-06-15
- Assignees: Apple Inc.
- Applicants: Apple Inc.
- Inventors: Vijay Phadke
- Web page: https://patentgrep.com/patent/US10770977B2

## CPC

- H02M 1/34: Snubber circuits
- H02M 3/01: {Resonant DC/DC converters}
- H02M 3/335: using semiconductor devices only
- H02M 3/33523: {with galvanic isolation between input and output of both the power stage and the feedback loop}
- H02M 3/33546: {with automatic control of the output voltage or current (H02M3/33561 takes precedence)}
- H02M 3/33569: {having several active switching elements (H02M3/3353 takes precedence)}
- H02M 3/33571: {Half-bridge at primary side of an isolation transformer}
- H02M 1/0009: {Devices or circuits for detecting current in a converter}
- H02M 1/0058: {by employing soft switching techniques, i.e. commutation of transistors when applied voltage is zero or when current flow is zero (using an auxiliary actively switched resonant commutation circuit connected to an intermediate DC voltage or between two push-pull branches of an inverter bridge H02M7/4811; in resonant inverters H02M7/4815; in inverters operating from a resonant DC source H02M7/4826)}
- H02M 1/342: {Active non-dissipative snubbers}
- Y02B 70/10: Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
- H02M: APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF (transformers H01F; dynamo-electric converters H02K47/00; controlling transformers, reactors or choke coils, control or regulation of electric motors, generators or dynamo-electric converters H02P)
- Y02B: CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS

## Abstract

Systems and methods for operating improved flyback converters are disclosed, in which leakage energy is returned to the input power source rather than to the output load, while still achieving zero voltage switching (i.e., ZVS) operation. In some embodiments, the improved converters may transfer the energy stored in the leakage inductance to a snubber capacitor(s) at the instant of turning off of the control switch. Further, the improved converter embodiments may also retain the stored energy in the snubber capacitor(s) when the power is being delivered to the load by the secondary circuits. The improved converter embodiments may start the transfer of leakage energy stored in the snubber capacitor(s) to the primary winding once the energy stored in the transformer is delivered to the load. Finally, the improved converter embodiments may intelligently control their active clamp switches such that all leakage inductance energy is returned to the input source.

## Claims

### Claim 1

A power conversion apparatus comprising:
a flyback converter, the flyback converter including a transformer having a primary winding, an auxiliary primary winding, and a secondary winding, and a first switch configured to be switched to alternately store energy in the transformer and transfer energy from the transformer to an output load; and
an active resonant clamp electrically coupled to the primary winding and the auxiliary primary winding, the active resonant clamp comprising:
a first diode coupled between the primary winding and the auxiliary primary winding and also coupled to the first switch;
a snubber capacitor coupled to the first diode and configured to receive leakage energy from the primary winding through the first diode;
a second diode coupled to the first diode and the snubber capacitor; and
a second switch coupled in series between the second diode and the auxiliary primary winding and configured to return the leakage energy from the snubber capacitor to the transformer.

### Claim 2 (depends on 1)

The power conversion apparatus of claim 1, wherein the auxiliary primary winding is electrically coupled to the primary winding.

### Claim 3 (depends on 1)

The power conversion apparatus of claim 1 wherein operation of the plurality of diodes is controlled at least in part by a reflected voltage appearing across the auxiliary primary winding.

### Claim 4

A method of operating a power conversion circuit comprising a flyback converter including a transformer having a primary winding, an auxiliary primary winding, and a secondary winding, and a first switch configured to be switched to alternately store energy in the transformer and transfer energy from the transformer to an output load and an active resonant clamp coupled to the auxiliary primary winding, the active resonant clamp including a second switch, at least two capacitors, and a plurality of diodes configured to recycle leakage energy from the transformer, the method comprising:
switching the first switch on, so as to store energy in a primary winding of the transformer;
switching the first switch off, so as to charge the at least two capacitors in series via the auxiliary primary winding, thereby recovering leakage energy from the transformer;
discharging the at least two capacitors in parallel to return the recovered leakage energy to the transformer.

### Claim 5 (depends on 4)

The method of claim 4 further comprising switching on the second switch when current is flowing through a body diode of the second switch.

### Claim 6 (depends on 4)

The method of claim 4, wherein discharging the at least one capacitor to return the recovered leakage energy to the transformer is started after all the energy stored in the transformer is delivered to the load.

### Claim 7

A power conversion apparatus, comprising:
a transformer having a primary winding, an auxiliary primary winding, and a secondary winding;
a first switch coupled to the primary winding so as to control a flow of current through the primary winding;
a second switch coupled to at least one of the primary winding and the auxiliary primary winding, so as to control a flow of current through an active resonant clamp circuit via the auxiliary primary winding,
wherein the active resonant clamp circuit is configured to absorb leakage energy from the primary winding, and comprises:
a first diode coupled between the primary winding and the auxiliary primary winding and also coupled to the first switch;
a snubber capacitor coupled to the first diode and configured to receive leakage energy from the primary winding through the first diode;
a second diode coupled to the first diode and the snubber capacitor; and
the second switch is coupled in series between the second diode and the auxiliary primary winding and configured to return the leakage energy from the snubber capacitor to the transformer.

### Claim 8 (depends on 7)

The power conversion apparatus of claim 7 further comprising a controller coupled to the first and second switches and configured to operate the first and second switches to:
alternately store energy in the transformer and transfer energy from the transformer to an output load; and
recover leakage energy from the transformer by charging the at least one capacitor via the auxiliary primary winding; and
return the recovered leakage energy to the transformer.

### Claim 9 (depends on 8)

The power conversion apparatus of claim 8 wherein:
the second switch is coupled to a first terminal of the auxiliary primary winding;
the at least one capacitor comprises first and second snubber capacitors; and
the plurality of diodes comprise a first diode coupled in series with the first and second snubber capacitors a second diode coupled in parallel with the first snubber capacitor and the first diode, a third diode coupled in parallel with the second snubber capacitor and the first diode;
wherein the active resonant clamp is configured to recover leakage energy from the transformer by charging the first and second snubber capacitors in series and return the recovered leakage energy to the transformer by discharging the first and second snubber capacitors in parallel.

### Claim 10 (depends on 9)

The power conversion apparatus of claim 9 wherein operation of the plurality of diodes is controlled at least in part by a reflected voltage appearing across the auxiliary primary winding.

### Claim 11 (depends on 7)

The power conversion apparatus of claim 7 wherein operation of the plurality of diodes is controlled at least in part by a reflected voltage appearing across the auxiliary primary winding.

### Claim 12

A power conversion apparatus comprising:
a flyback converter, the flyback converter including a transformer having a primary winding, an auxiliary primary winding, and a secondary winding, and a first switch configured to be switched to alternately store energy in the transformer and transfer energy from the transformer to an output load; and
an active resonant clamp coupled to at least one of the primary winding and the auxiliary primary winding, the active resonant clamp including a second switch, at least two capacitors configured to charge in series and discharge in parallel, and a plurality of diodes, the active resonant clamp being configured to recover leakage energy from the transformer.

### Claim 13 (depends on 12)

The power conversion apparatus of claim 12 wherein operation of the plurality of diodes is controlled at least in part by a reflected voltage appearing across the auxiliary primary winding.

### Claim 14 (depends on 12)

The power conversion apparatus of claim 12, wherein the auxiliary primary winding is electrically isolated from the primary winding.

### Claim 15 (depends on 14)

The power conversion apparatus of claim 14 wherein the active resonant clamp is electrically coupled to the auxiliary primary winding.

### Claim 16 (depends on 12)

The power conversion apparatus of claim 12 wherein:
the second switch is a clamp switch coupled to a first terminal of the auxiliary primary winding;
the at least two capacitors include first and second snubber capacitors;
the plurality of diodes include:
a first diode coupled in series with the first and second snubber capacitors;
a second diode coupled in parallel with the first snubber capacitor and the first diode; and
a third diode coupled in parallel with the second snubber capacitor and the first diode; and
wherein the active resonant clamp is configured to recover leakage energy from the transformer by charging the first and second snubber capacitors in series and return the recovered leakage energy to the transformer by discharging the first and second snubber capacitors in parallel.