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In order to reduce production costs, many manufacturers (including foreign manufacturers) have developed IP combination switching power supply boards that combine switching power supply boards and high-voltage backlights to provide high voltage to the backlight of the LCD screen while supplying power to the whole machine. powered by. The high-voltage power supply of the backlight tube generally adopts a high-voltage step-up transformer in order to make the switching power supply board not too complicated. The output power of the power amplifier circuit that drives this high-voltage step-up transformer is huge (to support 16 or even 24 backlight tubes). If the power amplifier circuit still uses 24V power supply, the power supply current is greatly increased, while the power tube is The on-resistance is constant, and an increase in current means that the voltage drop of the MOS tube is increased, which causes an increase in loss, an increase in heat generation, and a decrease in efficiency. The consequence: not only the increase in power consumption, but also the failure rate.

In order to overcome this problem, the power amplifying part needs to use a high voltage power supply of more than 24V, and the current does not increase in the case where the output power is greatly increased. Based on this consideration, a set of high-power single-ended push-pull output circuits are used, and the power supply voltage is +380V (the voltage output from the PFC power supply is directly supplied to reduce the burden of the PWM switching power supply) to accomplish this. This is a good method. Since there is only one high voltage transformer, it is more convenient to supply power to the EEFL backlight tube. For CCFL (external electrode fluorescent lamp) backlight tube, each backlight tube can be used to increase the balance inductance to solve the parallel connection. Application issues.

In the case of high voltage (380V) power supply, the P-channel + N-channel MOS transistor single-ended amplifier circuit is still not competent, mainly because the current voltage of the P-channel MOS transistor cannot work under high voltage. (N-channel UDS can reach 1000V, UDS of high-power P-channel MOS transistors can't even exceed 100V), and the gain is far less than that of N-channel MOS transistors. Therefore, it is only necessary to use a single-ended push-pull amplifier circuit (half-bridge architecture) composed of two high-power N-channel MOS transistors to accomplish this.

The figure below is a power amplifier circuit composed of two high-power N-channel MOS transistors. Since both are N-channel MOS transistors, the excitation signal is excited by two signals of opposite phase and equal amplitude.

Because of the high voltage power supply, the power is large, and the DC components of the two excitation signals are greatly different (the gate potential difference between the two power transistors is greatly different), and the excitation circuit is relatively complicated. At present, there are also foreign companies that specifically generate excitation integrated circuits for exciting N-channel + N-channel single-ended power amplifier circuits. For example, Fairchild's FAN7382 integrated circuit (below) uses a bootstrap circuit to solve the DC component of the signal. The design is unique.