Quasi-resonant, zero-current switching DC-DC converter

In order to reduce size and weight, switching converters with switching frequencies higher than the mains operating frequency appeared in the 1960s . Initially, the operating frequency of switching converters was between 20 kHz and 30 kHz. After the 1970s, with the promotion and application of advanced devices (such as high-speed transistors ), the switching frequency can reach more than 100 kHz. However, the switching loss increases with the increase of switching frequency, which seriously affects the performance of switching converters. In order to reduce switching losses, quasi-resonant, zero current switching (ZCS) DC-DC converters with switching frequencies up to 1 MHz have appeared. Each switching device is turned on and off at zero current, so the switching loss is only related to the on-current and has nothing to do with the switching frequency. In each switching cycle, the converter transmits high-frequency energy to the output.

Currently, switching converters are usually packaged into high- power density brick modules, as shown in Figure 1. When selecting DC-DC converter modules, power system designers usually only consider size, efficiency and price, but rarely consider circuit structure. Since there are many circuit structures (basic power conversion circuits) used in converters, understanding the circuit structure of the converter will help to select the appropriate converter.

Figure 1 - High-density DC-DC converter modules. There are thousands of possible combinations of converter modules, depending on input voltage, output voltage, and output power. The smallest module is shown here ; it measures 2.28 x 1.45 x 0.5 inches (57.9 x 36.8 x 12.7 mm) and has a maximum output power of 150 W.

This article mainly describes the circuit structure and working principle of quasi-resonant, zero-current switching DC-DC converters. The different characteristics and certain advantages of various circuit structures are also discussed.

Figure 2 is a simplified circuit diagram of a quasi-resonant, zero-current switching DC-DC converter. Since energy is transferred from the power supply to the load when a single solid-state switch is turned on, this converter is called a single-ended forward converter. This converter is a quasi-resonant converter, and the switch is switched at zero current, which truly eliminates switching losses. However, it is different from a resonant converter in that the energy stored in the capacitor Cr cannot be returned to the inductor Lr.

Figure 2 - Simplified circuit diagram of a quasi-resonant, zero-current switching DC-DC converter

The converter is mainly composed of the following components:

• Main switch Q1: When the switch is turned on, the current waveform flowing through the switch is close to a half-sine wave, transferring the energy of the input power supply to the LC circuit.
• Transformer T1: The transformer realizes primary-to-secondary voltage conversion and primary-to-secondary electrical isolation, as well as energy storage in the transformer leakage inductance, and plays an important role in this power architecture.
• LC circuit: The leakage inductance Lr of the transformer is connected in series with the secondary winding of the transformer as an inductor in Figure 2, and forms an LC circuit with the capacitor Cr. The energy storage formed by the current in the inductor is LrI2, and the energy storage formed by the voltage in the capacitor is CrV2.
• Dual diode output rectifier: Two diodes D1 and D2 force energy to transfer from the input to the output. When the main switch is on, diode D1 is on and energy is transferred from the leakage inductance to the capacitor Cr. Due to the rectifying effect of the diode, reverse energy transfer is prevented. When necessary, diode D2 can provide a current path for the output inductor LO. After the energy is transferred from the leakage inductance to Cr and LO, D2 also prevents reverse voltage from being applied to Cr.
• Core reset circuit: In practical applications, it is hoped that the core of the single-ended forward converter can be restored to its original state in each cycle. This can more effectively utilize the dynamic flux swing of the transformer core material, thereby allowing a core of a given size to transmit greater power.
• Low-pass filter: The main purpose of the output LC filter is to reduce the output ripple voltage across the load . The inductance of LO is large, and the energy stored in LO at full load current is greater than the energy stored in any other energy storage element in the circuit . In steady state operation, the energy delivered to the load must match the energy pulse delivered from Cr.

Quasi-resonant, zero-current switching DC-DC converters complete power conversion through energy transfer cycles. Under given input voltage conditions, each resonance transmits the same amount of power, and this energy can be transmitted at different repetition rates, so the total power (or voltage) transmitted to the output can be changed. This energy is then averaged or smoothed by the LO and Cr output filters to output a stable power (or voltage). When more power is required, the repetition rate will increase.

Although many converter circuits exist, the emergence of DC-DC converters as modular components requires some circuit configuration considerations. Due to certain inherent characteristics of switching converters, such as power consumption increasing with frequency, no single converter configuration is superior to the others in every aspect.

Unlike resonant converters, quasi-resonant converters can only transfer energy in one direction, from source to load, with high efficiency and inherent stability.

Zero current switching is enabled by coupling the leakage inductance of the power supply's main transformer with a resonant capacitor on the transformer's secondary side. The main switch turns on and off when the current passes through zero, thus eliminating the high di/dt current change rate and high switching noise that many other switching converters have. This sine wave "soft" switching also reduces component parasitic noise generated by square wave "hard" switching.

A core reset circuit can generate a mirror magnetizing current, which enables the converter to operate in the 1st and 3rd quadrants of the BH hysteresis loop, as shown in Figure 3. This magnetizing current has many operating advantages, such as the transformer core has the maximum magnetic flux and the main switch is subjected to the lowest voltage stress, so cheaper main switch devices with lower rated voltage can be selected. These advantages help improve the efficiency and power density of the converter, and at the same time, they can also reduce the cost of the converter. Other core reset circuits can only make the single-ended forward converter work in the first quadrant, so the duty cycle of the converter is subject to certain restrictions.

Figure 3 - The core reset circuit using mirror magnetizing current allows the core to operate in the 1st and 3rd quadrants, fully utilizing the core area. The converter can have a higher duty cycle and use a smaller reset switch, reducing power consumption.

In recent circuit configurations, converter features include wide trimming range and fault tolerant architecture. These converters can program the output voltage from 10% to 110% of the nominal voltage using fixed resistors, potentiometers or DAC voltages. For example, a 12 Vdc output can be trimmed over a range of 1.2 Vdc to 13.2 Vdc. These converters also have N+M's patented fault tolerant architecture. One module in a parallel array automatically controls startup and the other modules will work in sync with it. The modules in the array communicate with each other via high-speed pulses on the input bus . If the leading module fails, another module will automatically take over as the leader and the system will continue to operate without being affected.