What is a step down voltage regulator

A simple circuit that converts a high AC input voltage into a much lower DC voltage, such as can be used in energy meters.

In this power tip we want to look at a simple circuit that converts a high AC input voltage into a much lower DC voltage, as can be used in applications such as electronic energy meters.

In this special application, no electrical isolation between output and input voltage is required. Here the rectified AC input voltage can be up to 375 VDC; the output supplies a voltage in the order of magnitude of 5 V and can be loaded with currents of several hundred milliamps. These assemblies, which are manufactured in mass quantities, are often quite cost-sensitive, so that a circuit is required which manages with few components and is as inexpensive as possible.

A step-down voltage regulator is an inexpensive solution; however, implementing them with a high voltage input can present a number of challenges for the designer. In continuous operation, the voltage ratio of this step-down converter is calculated by dividing the output voltage by the input voltage, which results in a value of 1.25% for a conversion from 400 V to 5 V. If the power supply circuit is operated with a switching frequency of 100 kHz, the switch-on time must be 125 ns, which is often impractical due to restrictions with regard to the switching speed.

Figure 1: A simple and inexpensive bias power supply with a low voltage buck converter IC

Figure 1 shows a circuit that meets the challenge with regard to the duty cycle. A constant-on-time controller (U1) controls a high-voltage step-down converter power stage, which consists of a p-channel FET (Q4) and is controlled by a potential converter circuit (Q2, Q3) to convert 400 V into the desired low voltage of To convert 5 V. The controller (in this example a TPS64203) forms the core of this assembly. It has a very low quiescent current consumption (35 µA) so that the converter can start up in offline mode and only minimal power losses occur in the resistors R2 and R3.

The second important aspect of this circuit is its ability to deliver short (600 ns) on-time gate drive pulses to increase the minimum switching frequency (in CCM operation) to values ​​above 20 kHz. Q1 is used to shift the gate drive voltage to the potential of the high-voltage side driver. With a low signal at the IC output, approx. 5 V are applied to R4, which means that a fixed current flows through Q1 and R5. The voltage across R5 arrives at the gate of the p-channel FET via emitter followers. The current simultaneously charges the capacitor C4, so that the driver circuit receives operating voltage.

The p-channel FET was chosen to simplify the driver circuit. If an n-channel type is to be used, a circuit variant would be required to apply a voltage above the input voltage to the gate of the FET and thus to fully control the component.

Fig. 2: The MOSFET has short switching times (<50 ns)

Figure 2 shows two signal curves that show that short switching times can be achieved with the simple bipolar drivers. The gate drive voltage rise and fall times of less than 50 ns result in drain switching times of less than 30 ns. The switching times can be shortened by optimizing the control current flowing to the p-channel FET, but this is at the expense of increased power loss.

The efficiency of this circuit is about 70%. This is a very good value when you consider that the power is only 4 W, a conversion from 400 V to 5 V takes place and the circuit is simple and inexpensive. Two weaknesses of this circuit are that it has neither short-circuit nor overvoltage protection. Nevertheless, the circuit is likely to represent a cost-effective compromise in many applications.

OfRobert Kollman, Texas Instruments