Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

[Introduction]Offline flyback power supplies require a clamping circuit (sometimes called a snubber) on the primary side of the transformer to limit the drain-source voltage stress across the power MOSFET switch when it is turned off during normal operation. Different approaches can be used when designing a clamp circuit.

Offline flyback power supplies require a clamping circuit (sometimes called a snubber) on the primary side of the transformer to limit the drain-source voltage stress across the power MOSFET switch when it is turned off during normal operation. Different approaches can be used when designing a clamp circuit. Low-cost passive networks can effectively implement voltage clamping, but the clamping energy must be dissipated during each switching cycle, which reduces efficiency. One way to improve is to use complementary drive active clamp techniques for the clamps and power switches, resulting in improved energy efficiency, but they impose limitations on the mode of operation of the power supply (eg, inability to operate in CCM mode). To overcome the design limitations imposed by complementary active clamp circuits, another more advanced control technique, non-complementary active clamp, can be used. This technique ensures that the clamping energy is used in a more cost-effective manner.

This article will briefly describe the need for a primary clamp circuit in a flyback power supply. Then compare and contrast the use of passive clamp scheme, complementary active clamp scheme and non-complementary active clamp scheme, and finally introduce a chip that supports non-complementary clamp scheme and can realize ultra-high power density flyback power supply design Group.

In a flyback converter, when the primary side switch is turned off, the voltage (VOR) is reflected from the secondary side to the primary side, and the stored energy is transferred through the transformer to the load (Figure 1). VOR is amplified by the transformer turns ratio, and superimposed on the VDC input bus voltage will increase the voltage stress across the switching device. In conventional circuits, passive primary clamps are used to limit this voltage.

Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

Figure 1: Passive primary clamp RCD solutions (highlighted) dissipate a lot of heat, limiting the efficiency and operating frequency of the flyback power supply

In addition to the voltage stress (VIN + VOR), a large voltage overshoot occurs when the primary switch is turned off, which is caused by the energy stored in the leakage inductance of the primary winding. A clamp circuit protects the primary switch by limiting the voltage overshoot from these three factors (Figure 2). Furthermore, with this circuit configuration, the power switch turns on when the drain voltage is high. Switching losses are proportional to VDS2, so a high VDS increases the turn-on losses of the switch, further reducing efficiency.

Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

Figure 2: Both turn-on and clamping losses are related to switching frequency.

The clamp capacitor absorbs leakage inductance energy, but this energy is then dissipated by the clamp resistor. There is energy loss in each switching cycle, which limits the increase in switching frequency in practice. For lower switching frequencies, larger transformers are required. Therefore, using passive clamps increases losses and has to use lower switching frequencies, both of which increase the size of the power supply. Using active clamps overcomes these limitations.

Complementary Active Clamp

Active clamps replace the resistors in the RCD clamp with a switch, usually a power MOSFET (Figure 3). Instead of dissipating leakage inductance energy, it can transfer leakage inductance energy back to the transformer. In complementary active clamping, when the main MOSFET is turned off, the clamp switch is turned on with a small dead time in between. At this point the clamp capacitor is charged. Before the next main MOSFET is turned on, the clamp switch is turned off and the energy in the clamp capacitor can be recycled to the output. This active clamp is called a complementary drive scheme because the main MOSFET and active clamp switch work in a complementary fashion.

Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

Figure 3: Typical[互补]Simplified Schematic of Active Clamp Scheme

Zero-voltage switching can be implemented using complex adaptive control techniques to achieve resonance between leakage inductance and clamp capacitance. When the clamp switch is turned off, the negative current generated by the resonance of the leakage inductance and the clamp capacitor discharges the voltage across the COSS before the power MOSFET is turned on, thereby realizing zero-voltage switching. For designs with high output capacitance, the resonance effect will be deteriorated (the output capacitance will be reflected to the primary through the transformer, thereby increasing the capacity of the clamping capacitor). Usually there will not be enough leakage inductance energy storage in the transformer to accommodate this change in clamping capacity. To overcome this problem, a two-stage LC filter is often required at the power supply output to ensure low primary reflected capacitance while still meeting output ripple requirements. This complementary active clamping scheme is an improvement over passive clamping, but still suffers from the following limitations:

1. Requires burst mode at light load, which results in higher output ripple

2. Two-stage output filter

3. Limited to critical conduction mode or discontinuous conduction mode (CrM and DCM); no CCM mode of operation, making USB PD designs with wide output voltage ranges difficult to achieve

Improve performance with non-complementary active clamps

With a non-complementary control scheme, instead of turning on the clamp switch a short time after the main MOSFET is turned off, the clamp switch is turned on briefly before the main MOSFET is turned on. Non-complementary control is able to operate in continuous conduction mode as well as discontinuous conduction mode (and CrM) and still achieve zero voltage switching. This enables power supplies to be designed with a very wide input voltage range and a wide output voltage range, which is required for designing efficient USB PD chargers. For traditional control schemes, the synchronization design of the drive signal of the non-complementary clamp switch with the primary switch and the synchronous rectifier switch faces challenges. Using a single controller to manage the switching operations of all three devices greatly simplifies the circuit and ensures reliable operation.

Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

Figure 4: For non-complementary mode switches, the active clamp switch switches only once before the main switch turns on

Non-complementary active clamp control can be implemented using Power Integrations’ Innoswitch™ 4-CZ/ClampZero™ chipset (Figure 5). Housed in an InSOP™-24D package, the InnoSwitch4-CZ device integrates a robust PowiGaN™ 750V switch and a secondary controller for main switch, clamp switch, and synchronous MOSFET operation, as well as a safety-compliant FluxLink™ control link. The InnoSwitch4-CZ IC includes two pins dedicated to ClampZero active clamp non-complementary control: the high-side drive (HSD) pin for turning the ClampZero switch on and off, and the V pin for measuring the DC bus voltage foot.

Non-Complementary Active Clamp Enables Ultra-High Power Density Flyback Power Supply Designs

Figure 5: The HSD signal of the InnoSwitch4-CZ is used to control the switch of the ClampZero active clamp,

The V pin is used to detect a high input voltage condition to enable discontinuous operation

The secondary-side controller issues a command to activate the HSD signal to turn on the ClampZero PowiGaN switch to resonate the leakage inductance and clamp capacitance before the primary PowiGaN switch is commutated. There is a very small delay between the turn-off of the ClampZero device and the turn-on of the main switch, which can be adjusted externally with a small resistor on the HSD pin to help optimize timing.

In continuous conduction mode, the HSD signal remains on for one quarter of the resonant period of the leakage inductance and the clamp capacitor. One challenge in using this resonant mode over a wide operating range is that the leakage inductance is usually a very small value, and the voltage across the main switch is higher at high input conditions, which requires more energy achieve zero voltage switching. Therefore, the energy storage of leakage inductance is often insufficient. This is also the reason why the discontinuous conduction control mode needs to be involved at this time.

For discontinuous conduction mode (high input voltage operation), the HSD signal pulse width becomes the magnetizing inductance (plus the leakage inductance, although the component of leakage inductance is usually very small compared to the magnetizing inductance) and the clamping capacitor produces the resonant period of the resonance a quarter of the time. The input voltage information on the V pin is used to control the initiation of discontinuous conduction mode. The delay between the ClampZero turn-off drive signal and the main switch turn-on drive signal also increases when a high input voltage condition is detected. This provides more time for resonance between the magnetizing inductance (plus leakage inductance) and the clamping capacitor to reduce the voltage across the main power switch. This mode of operation does not require the burst mode of operation required by complementary active clamp circuits, avoiding the risk of higher output ripple and audible noise from complementary mode control.

Summarize

Off-line flyback power supplies require the use of a primary-side clamp circuit to protect the power MOSFETs. Using passive RCD clamps is low cost but lower performance. Using active clamps with complementary control schemes can improve performance, but limitations remain. The InnoSwitch4-CZ IC product family provides a unique control architecture that enables more complex non-complementary active clamp control, enabling the design of very wide input voltage ranges and large output voltage set point variations. Efficient, ultra-compact USB PD charger. Power Integrations’ InnoSwitch4-CZ/ClampZero chipsets can be used to simplify active clamping schemes using non-complementary control and speed time to market.

(Source: Power Integrations, by Roland Saint Pierre, Director of New Product Definition, Power Integrations)

The Links:   LQ084V3DG01 CM600DU-24FA DISPLAY-COMPANY

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