Power integrations InnoSwitch3 Application Note Download Page 23

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Rev. A 10/18

Application Note

AN-72

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close as possible to the SOURCE and PRIMARY BYPASS pins of the

device.

Primary sensed OVP can be realized by connecting a

series

combination of a Zener diode, a resistor and a blocking diode from

the rectified and filtered bias winding voltage supply to the PRIMARY

BYPASS pin (see Figure 18-a). The rectified and filtered bias winding

output voltage may be higher than expected (up to 1.5X or 2X the

desired value) dependent on the coupling of the bias winding with the

output winding and the resulting ringing on the bias winding voltage

waveform. It is therefore recommended that the rectified bias

winding voltage be measured. Ideally this measurement should be

made at the lowest input voltage and with full output load. This
measured voltage should be used to select the components required
to provided primary sensed OVP. It is recommended that a Zener
diode with a clamping voltage approximately 6 V lower than the bias
winding rectified voltage at which OVP is expected to be triggered be
used. A forward voltage drop of 1 V can be assumed for the blocking

diode. A small signal standard recovery diode should be used. The

blocking diode prevents any reverse current charging the bias
capacitor during start-up. Finally, the value of the series resistor set

such that a current higher than I

will flow into the PRIMARY BYPASS

pin during an output overvoltage event.

Secondary-Side Overvoltage Protection

Secondary-side output overvoltage protection is provided by the
InnoSwitch3 IC. It is activated when an internal auto-restart is

triggered when a current exceeding the I

BPS(SD)

threshold is fed into

the SECONDARY BYPASS pin. The direct output sensed OVP function
can be realized by connecting a Zener diode from the output to the
SECONDARY BYPASS pin. The Zener diode voltage rating shall be the
difference between 1.25 V

OUT

and 4.4 V SECONDARY BYPASS pin

voltage. It is necessary to add a low value resistor, in series with the

OVP Zener diode to limit the maximum current into the SECONDARY
BYPASS pin (see Figure 18-b).

An OVP for a 5 V output can be implemented by two-diodes in series
(shown in Figure 18-c). The filter capacitor should be rated for 6.3 V.

Recommendations for Circuit Board Layout

Single-Point Grounding

Use a single-point ground connection from the input filter capacitor to

the area of copper connected to the SOURCE pin. See Figures 19 and

20.
Bypass Capacitors

The PRIMARY BYPASS (C

BPP

), SECONDARY BYPASS (C

BPS

) decoupling

capacitors must be located directly adjacent to the PRIMARY
BYPASS-SOURCE, SECONDARY BYPASS-GROUND and FEEDBACK-
GROUND (C

) pins respectively and connections should be routed via

short traces.

Signal Components

External components R

, R

FB(UPPER)

, R

FB(LOWER)

and R

which are

used for monitoring feedback information must be placed as close as

possible to the IC pin with short traces.

Critical Loop Area

Circuits where high dv/dt or di/dt occurs should be kept as small as
possible. The area of the primary loop that connects the input filter
capacitor, transformer primary and IC should be kept as small as

possible.

No loop area should be placed inside another loop (see Figure 21).
This will minimize cross-talk between circuits.

Primary Clamp Circuit

A clamp is used to limit peak voltage on the DRAIN pin at turn-off.
This can be achieved by using an RCD clamp or a Zener diode (~200 V)
and diode clamp across the primary winding. To reduce EMI, minimize
the loop between the clamp components, the transformer and the IC.

PI-8466a-111017

Secondary

Control IC

OUT

RTN

FB(UPPER)

FB(LOWER)