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PS18/28/35 Troubleshooting Guide

Theory of Operation

The IC has two enable pins, one of which (pin-8) disables the operation while the other (pin-9)

enables the operation. A voltage pulse higher than 0.6V at pin-8 shuts down the IC and a voltage

pulse higher than 1.2V at pin-9 wakes up the IC. An opamp, an under voltage lockout circuit, and a

soft start circuit are also built into the IC.

Figure 3 is a circuit diagram that shows how the IC is used in the tracking power supply. With a

12V voltage established at pin-12 (Vs), the VCO starts to oscillate, sweeping oscillation frequency

from its start frequency (350 kHz) downward. A gate-driving signal for a low-side MOSFET is

generated at pin-11 with respect to pin-10, and another gate driving signal for a high-side

MOSFET is obtained at pin-15 with respect to pin-14.

Figure 3. Resonant controller IC used in tracking power supply

Figure 4 shows the oscillograms of the two gate driving signals, when the IC alone is powered on

by a 12V DC supply. A dead time of about 280ns, during which both signals are at low level, exists

between the two gate signals. Such a dead time avoids shoot-through of the two power MOSFETs.

It also allows zero-voltage switching as will be explained later.

The frequency of the two signals is controlled by a voltage at pin-7, which is connected to the

output of an opto-coupler. The frequency is about 200 kHz with 0V at pin-7 and 100 kHz with 2V at

pin-7.

Figure 4. Scope photo of two gate driving signals