Floating Constant Current Source Allows Ultra-Wide Input Voltage Range
For many Power Integrations products, the datasheet limit for minimum drain voltage to guarantee proper startup and functionality is 50 V. However if current is externally fed into the BYPASS pin, the chip can be powered externally and can start up and operate at even lower input voltages.
Figure 1. Floating constant current source circuit to power controllers
The start-up circuit shown in Figure 1 is a floating constant current source and provides a constant current of about 600 µA into the BP pin of TinySwitch-III over the complete input voltage range.
R2 together with VR1 determines the constant current value:
The circuit is derived from the basic single transistor current source, which uses a zener to set reference voltage for base terminal of Q2 (NPN), which in turn programs the fixed voltage across resistor R2, thus setting the constant current value. However, given the very wide range of the input supply, the bias current for the reference zener diode varies over a large range. This causes both increased power dissipation as well as shifting of the programmed constant current.
To overcome this difficulty, the bias current is provided by another current source formed by Q1 (PNP) and R1. A constant voltage equal to VBE is imposed across R1, which fixes the bias current into the reference zener throughout the operating range.
Transistor Q2 delivers the constant current at lower input voltages and Q1 at higher input voltages. Figure 2 shows the simulation results of the current flow through Q1 and Q2. Up to an input voltage of about 50 VDC, the constant current is supplied by Q2. From 50 VDC and above, the current through Q2 will decrease, and the current through Q1 linearly increases. At the maximum input voltage of 375 VDC, the constant current is provided primarily by Q1.
R3 is used to limit the input current of the whole circuit at maximum input voltage.
Figure 2. Transistor currents and total BP pin current over input voltage
The non-linear current rise comes from the non-linear behavior of the Zener diode VR1. At about 60 VDC input voltage, the Zener voltage is reached.