Principle and application of three-terminal monolithic switching power supply TOP412/414

    Abstract: The three-terminal monolithic switching power supply TOP412/414 is an integrated control chip produced by the American Powergration Company that integrates a PWM controller and a MOSFET power switch. This article introduces the performance characteristics and working principle of the chip, and gives a typical application circuit.

    Keywords: TOPSwitch pulse width modulation monolithic switching power supply TOP412/414

1 Introduction

TOPSwitch412/414 series devices are three-terminal isolated pulse width modulation monolithic switching power supply integrated circuits produced by Powergration Company in the United States. This device integrates a PWM integrated circuit and a MOSFET power device into the same chip and has all the functions required for a PWM switching regulated power supply. At the same time, it also contains an on-state controllable gate drive circuit, a high-voltage N-channel off field effect transistor, and a 12kHz oscillator. Circuit-type PWM modulator, high-voltage start-up bias circuit, bandgap reference, shunt regulator for loop compensation, error amplifier and fault protection and other functions. This device completely isolates the output end from the power grid through a high-frequency transformer, thereby achieving monolithic integration of a switching power supply isolated by a reactive frequency transformer. TOPSwitch412/414 is packaged in SMD-8 and uses 6 pins to transfer heat directly to the circuit board, thus reducing the cost of the heat sink.

●Low cost;

●High system efficiency;

●Simple design;

●Has system-level fault protection function;

●Has strong application flexibility;

2 Internal structure

TOPSwitch412/414 adopts SMD-8 package, and the 8 pins can be simplified into 3 pins, namely source S, drain D and control electrode C. Among them, the source is actually the source connected to the internal MOSFET. It is the common point of the primary circuit and the reference point of the power return flow. The drain is the drain connected to the internal MOSFET. When the circuit is started, the internal bias current is provided by the internal high-voltage switching current source.

The control pole in the chip is the input terminal of the error amplifier circuit and the control duty cycle feedback current. During normal operation, the internal bias current is provided by the internal shunt regulator. It can stimulate input current when the system is shut down, and can also serve as a capacitive connection point to provide bypass, auto-restart and compensation functions.

Control voltage source: The control voltage Vc provides bias voltage to the parallel regulator and gate driver, while the control terminal current Ic adjusts the duty cycle.

Bandgap reference voltage source: The quasi-voltage of the bandgap with temperature compensation is used to provide various reference voltages for internal circuits and generate MOSFET gate drive current.

Oscillator: An internal oscillator generates the sawtooth waveform required for the PWM ratio by linearly charging and discharging an internal capacitor in two voltage ranges. The oscillation frequency is fixed at 120kHz, which helps reduce EMI and improve efficiency in power supplies.

PWM modulator: By changing the size of the control terminal current Ic, the voltage drop of the resistor RE can be changed. After RC filtering, the PWM comparator compares it with the sawtooth wave to generate a PWM pulse width modulation signal to drive the MOSFET tube. Its pulse width is inversely proportional to the current Ic at the control end.

Gate drive circuit: The gate drive circuit can turn on the MOSFET at a certain rate to reduce EMI. For improved accuracy, the gate drive can also implement cycle-by-cycle current limiting.

Error Amplifier: The error amplifier's non-inverting termination provides a precision quasi-voltage from a bandgap voltage reference. The inverting terminal is connected to the control electrode feedback voltage, and its gain is determined by the dynamic impedance of the control electrode.

Overcurrent protection: This device uses the MOSFET's drain-source on-state resistance as a sensor resistor to detect current. In this way, when the drain current is too large, overcurrent protection can be achieved by turning off the MOSFET.

Shutdown/auto-restart current: To reduce power consumption, the shutdown/auto-restart circuit turns power on and off at a 5% duty cycle when the circuit exceeds regulation. After troubleshooting, the power supply will automatically restart to resume normal operation.

Protection circuit: When the chip junction temperature exceeds 145°C, the output shuts down. In addition, TOPSwitch412/414 also has under-voltage and over-current protection functions.

3 Basic working principles

The open-drain output TOPSwitch chip is a self-biased, self-protected current-duty cycle linear control converter. The control electrode voltage Uc can provide bias voltage or power supply for the controller and driver. By connecting an external bypass capacitor in parallel between the source of the control electrode, the shutdown/automatic restart time can be set for all capacitors CT connected to the control electrode.

During startup, the internal high-voltage current source connected between the drain and the source charges the control electrode, because the soft start of the circuit can be achieved through the charging process of the capacitor external to the control electrode. When Uc reaches 5.7V, the internal high-voltage current source is turned off, and the feedback control current supplies power to Uc. In the normal working stage, the external circuit forms a voltage negative feedback control loop to adjust the duty cycle of the output stage MOSFET to achieve voltage stabilization. When the output voltage increases, Uc increases, and the error voltage on the sampling resistor RE also increases. After comparing with the sawtooth wave, the duty cycle of the output voltage will be reduced, thereby reducing the voltage of the switching power supply. When the control electrode voltage is lower than 4.7V, the MOSFET tube is turned off, the control circuit is in a small current waiting state, and the internal high-voltage current source is reconnected and charges Uc. Its shutdown/auto-restart hysteresis comparator can protect Uc between 4.7~5.7V. Figure 2 shows its operating waveform, where a is the normal operating waveform and b is the automatic restart waveform.

The auto-restart circuit has a divide-by-eight counter that prevents the output stage MSOFET from turning on again until the first discharge-charge cycle is completed. Therefore, during automatic restart, controlling the duty cycle to about 5% can effectively limit the power consumption of the chip.

4 Typical applications

Figure 3 is a 5V/2A switching regulated power supply designed using TOPSwitch414. Its input voltage range is 36~72VDC.

VR1 and D1 connected in parallel on the primary side of the transformer are buffer circuits to eliminate the reverse voltage formed when the primary side of the transformer is turned off. C3~C6 and L1 form a π-type high-frequency filter. Linear optocoupler PC317 and adjustable precision shunt regulator TL431 provide a closed-loop negative feedback loop. When the +5V voltage changes, the error voltage formed by comparing the sampled voltage divided by resistors R3 and R4 with the 2.5V voltage provided by TL431 will cause corresponding changes in the LED's operating current, and will be controlled by the optocoupler U2. According to the size of the pole current Ic, adjusting the output duty cycle of TOPSwitch414 can keep the power supply output voltage unchanged to achieve the purpose of voltage stabilization.