# US 12,113,448 B2 · Zero-voltage-switching flyback converter with reduced secondary side current and voltage stress

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- Number: US 12,113,448 B2
- Document ID: US12113448B2
- Status: Granted
- Kind: B2
- Application number: 17/443,890
- Filed: 2021-07-28
- Published: 2022-02-03
- Granted: 2024-10-08
- Earliest claimed: 2020-07-28
- Assignees: Dialog Semiconductor (UK) Limited
- Applicants: Dialog Semiconductor (UK) Limited
- Inventors: Huaming Guo; Yi Xu; Zhaowu Luo; Yongcai Lai; Jianming Yao
- Web page: https://patentgrep.com/patent/US12113448B2

## CPC

- H02M 3/33592: {having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer}
- H02M 1/0025: {Arrangements for modifying reference values, feedback values or error values in the control loop of 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

A flyback converter is provided in which either auxiliary winding or the primary winding is split into two windings. In this fashion, a different turn ratio is presented to the secondary winding during the transformer reset period as compared to when an active-clamp transistor or a ZVS switch transistor is switched on.

## Claims

### Claim 1

A zero-voltage switching (ZVS) flyback converter, comprising:
a transformer having a primary winding, a secondary winding, a first auxiliary winding, and a second auxiliary winding;
an SR switch transistor in series with the secondary winding;
a first diode;
an auxiliary capacitor in series with the first auxiliary winding and the first diode, wherein the first diode is configured to allow a charging current to flow from a positive terminal of the first auxiliary winding into a positive terminal of the auxiliary capacitor and to prevent a current flow into the positive terminal of the first auxiliary winding;
a second diode; and
a ZVS switch transistor in series with the second auxiliary winding and the second diode, wherein the second diode is configured to allow a discharging current to flow from the positive terminal of the auxiliary capacitor through the second auxiliary winding while the ZVS switch transistor is on and to prevent a current flow through the second auxiliary winding while the charging current is conducting.

### Claim 2 (depends on 1)

The ZVS flyback converter of claim 1, wherein the ZVS switch transistor is an NMOS switch transistor.

### Claim 3 (depends on 1)

The ZVS flyback converter of claim 1, wherein the ZVS switch transistor is PMOS switch transistor.

### Claim 4 (depends on 1)

The ZVS flyback converter of claim 1, further comprising:
a power switch transistor coupled to the primary winding; and
a primary-side controller for switching on the ZVS switch transistor during a DCM mode of operation after the power switch transistor has cycled off and before the SR switch transistor has cycled on to cause a voltage across the power switch transistor to discharge toward zero volts.

### Claim 5 (depends on 2)

The ZVS flyback converter of claim 2, wherein a negative terminal of the auxiliary capacitor is coupled to ground, the positive terminal of the auxiliary capacitor is coupled to the positive terminal of the first auxiliary winding and coupled to a positive terminal of the second auxiliary winding, an anode of the first diode is coupled to ground, a cathode of the first diode is coupled to a negative terminal of the first auxiliary winding, an anode of the second diode is coupled to a negative terminal of the second auxiliary winding, a cathode of the second diode is coupled to a drain of the ZVS switch transistor, and a source of the ZVS switch transistor is coupled to ground.

### Claim 6 (depends on 2)

The ZVS flyback converter of claim 2, wherein a negative terminal of the auxiliary capacitor is coupled to ground, the positive terminal of the auxiliary capacitor is coupled to a positive terminal of the first auxiliary winding and to a source of the ZVS switch transistor, a drain of the ZVS switch transistor is coupled to a positive terminal of the second auxiliary winding, an anode of the first diode is coupled to ground, a cathode of the first diode is coupled to a negative terminal of the first auxiliary winding, an anode of the second diode is coupled to a negative terminal of the second auxiliary winding, and a cathode of the second diode is coupled to ground.

### Claim 7 (depends on 1)

The ZVS flyback converter of claim 1, wherein a number of turns for the first auxiliary winding is less than a number of turns for the second auxiliary winding.

### Claim 8 (depends on 1)

The ZVS flyback converter of claim 1, further comprising:
a primary-side controller for switching on the ZVS switch transistor while the SR switch transistor is on during CCM operation to signal to an SR controller to switch off the SR switch transistor.

### Claim 9 (depends on 8)

The ZVS flyback converter of claim 8, wherein a number of turns for the first auxiliary winding is greater than a number of turns for the second auxiliary winding.

### Claim 10

A zero-voltage switching (ZVS) flyback converter, comprising:
a transformer having a primary winding, a secondary winding, a first auxiliary winding, and a second auxiliary winding;
an SR switch transistor in series with the secondary winding;
a first diode;
an auxiliary capacitor in series with the first auxiliary winding and the first diode, wherein the first diode is configured to allow a charging current to flow from a positive terminal of the first auxiliary winding into a positive terminal of the auxiliary capacitor;
a second diode; and
a ZVS switch transistor in series with the first auxiliary winding, the second auxiliary winding and the second diode, wherein the second diode is configured to allow a discharging current to flow from the positive terminal of the auxiliary capacitor through the first auxiliary winding and through the second auxiliary winding while the ZVS switch transistor is on.

### Claim 11 (depends on 10)

The ZVS flyback converter of claim 10, wherein the positive terminal of the auxiliary capacitor is coupled to the positive terminal of the first auxiliary winding, the second auxiliary winding has a positive terminal coupled to a negative terminal of the first auxiliary winding, the first diode has an anode coupled to a negative terminal of the auxiliary capacitor and has a cathode coupled to the negative terminal of the first auxiliary winding and to the positive terminal of the second auxiliary winding, and wherein the second diode is coupled between the ZVS switch transistor and a negative terminal of the second auxiliary winding.

### Claim 12 (depends on 11)

The ZVS flyback converter of claim 11, wherein an anode of the second diode is coupled to a negative terminal of the second auxiliary winding, a cathode of the second diode is coupled to a drain of the ZVS switch transistor, and a source of the ZVS switch transistor is coupled to ground.

### Claim 13 (depends on 10)

The ZVS flyback converter of claim 10, wherein the positive terminal of the auxiliary capacitor is coupled to the positive terminal of the first auxiliary winding, the second auxiliary winding has a negative terminal coupled to a negative terminal of the auxiliary capacitor and has a positive terminal coupled to a source of the ZVS switch transistor, and wherein the second diode is coupled between a drain of the ZVS switch transistor and a negative terminal of the first auxiliary winding.

### Claim 14 (depends on 13)

The ZVS flyback converter of claim 13, wherein the first diode has an anode coupled to the negative terminal of the auxiliary capacitor and a cathode coupled to the negative terminal of the first auxiliary winding, and the second diode has an anode coupled to the negative terminal of the first auxiliary winding and a cathode coupled to a drain of the ZVS switch transistor.

### Claim 15

A method of operation for a ZVS flyback converter, comprising:
switching on an SR switch transistor coupled to a secondary winding of a transformer for an SR switch transistor on-time period;
charging an auxiliary capacitor to a peak voltage by conducting a charging current through a first auxiliary winding of the transformer responsive to the switching on of the SR switch transistor; and
switching on a ZVS switch transistor for a ZVS switch transistor on-time period to cause the auxiliary capacitor to discharge from the peak voltage by conducting a discharging current from the auxiliary capacitor to ground through a second auxiliary winding of the transformer.

### Claim 16 (depends on 15)

The method of claim 15, wherein the switching on of the ZVS switch transistor causes a voltage across the secondary winding of the transformer to be less than an output voltage of the ZVS flyback converter.

### Claim 17 (depends on 15)

The method of claim 15, wherein the charging of the auxiliary capacitor to the peak voltage also includes comprises conducting the charging current through the second auxiliary winding.

### Claim 18 (depends on 15)

The method of claim 15, wherein the switching on a ZVS switch transistor for the ZVS switch transistor on-time period to cause the auxiliary capacitor to discharge from the peak voltage also includes conducting the discharging current through the second auxiliary winding.

### Claim 19

An active-clamp flyback converter, comprising:
a transformer having a primary winding, an auxiliary winding, and a secondary winding;
an active-clamp capacitor;
an input voltage rail coupled to a negative terminal of the primary winding and to a negative terminal of the active-clamp capacitor;
a power switch transistor coupled between a positive terminal of the primary winding and ground;
an active-clamp transistor coupled between the positive terminal of the primary winding and a negative terminal of the auxiliary winding, the auxiliary winding having a positive terminal coupled to a positive terminal of the active-clamp capacitor; and
a diode coupled between the positive terminal of the primary winding and the positive terminal of the auxiliary winding.

### Claim 20 (depends on 19)

The active-clamp flyback converter of claim 19, wherein the diode has an anode coupled to the positive terminal of the primary winding and a cathode coupled to the positive terminal of the auxiliary winding.