Floating constant current source allows ultra-wide input voltage range

For most Power Integrations products, the data sheet limits the minimum drain voltage to 50 V to ensure proper startup and function. However, if the BYPASS pin is fed with an external supply, the chip can be powered externally and start and operate even at lower input voltages.

Figure 8: Floating constant current source circuit for power controller

The startup circuit shown in Figure 8 is a floating constant current source that provides a constant current of approximately 600 μA to the BYPASS (BP) pin of TinySwitch-III over the entire input voltage range.

The constant current value is determined by R2 and VR1:

This circuit is derived from a basic single-transistor current source. The circuit uses a Zener diode to set a reference voltage for the base terminal of Q2 (NPN), which is used to program a fixed voltage through resistor R2 to set the constant current value. However, due to the extremely wide input supply range, the bias current of the reference Zener diode will vary over a wide range. This will result in increased power dissipation and an offset in the programmed constant current.

To overcome this problem, an additional current source (formed by Q1 (PNP) and R1) is required to provide bias current. A constant voltage equal to VBE is forced across R1, which provides bias current compensation for the reference Zener diode over the entire operating range.

Transistor Q2 provides constant current at lower input voltages, while Q1 provides constant current at higher input voltages. Figure 2 shows the simulation results for current flowing through Q1 and Q2. When the input voltage reaches approximately 50 VDC, Q2 will provide constant current. When the input voltage reaches 50 VDC and above, the current through Q2 will decrease, while the current through Q1 will increase linearly. When the input voltage reaches the maximum value of 375 VDC, the constant current is mainly provided by Q1.

R3 is used to limit the input current of the entire circuit when the input voltage is maximum.

Figure 9: Transistor current vs. total BYPASS (BP) pin current when input voltage is exceeded The
non-linear current rises due to the non-linear action of Zener diode VR1. At input voltage of approximately 60 VDC, the Zener diode begins to have voltage.