STK730C

Overvoltage Protection Circuit Recommendations

When constant voltage power supply circuits fail, or a problem such as poor soldering on the PC board appears, the

failure mode can involve an overvoltage state in which the output voltage becomes higher than the specified voltage.

When an overvoltage failure occurs, the load driven by the power supply is often damaged, and severe damage can

occur. Also, if the overvoltage exceeds the output capacitor’s voltage limitation, the capacitor’s internal electrolytic fluid

can be vaporized and released by the operation of the capacitor’s explosion prevention valve. This gas can appear to be

white smoke.

When designing a power supply circuit, the need for an overvoltage protection circuit is the same regardless of whether

discreet components or integrated circuits are used. We strongly recommend the use of an overvoltage protection circuit

to hold to a minimum any damage that may result from an overvoltage state.

Overvoltage Protection Circuit Example

The crowbar circuit is a well known overvoltage protection circuit.

A Zener diode with a voltage 1 V to 2 V higher than the output voltage of the power supply is used as the diode ZD.

When the output voltage exceeds this value due to a malfunction or other cause, the Zener diode operates and the SCR

goes to the turn on short state since the SCR gate potential is pulled up. At this point the fuse at the input to the constant

voltage blows, completing the operation of the overvoltage protection circuit.

If the ambient temperature varies widely the crowbar circuit operating point can change due to the temperature

characteristics of both the Zener diode and the SCR’s cathode/gate voltage. As a result, the protection function may

operate even when the output voltage is normal. This circuit may also operate due to external noise.

Since, in general, the output voltage will be highest for the largest input and the smallest load, the operation and

effectiveness of the protection circuit must be confirmed in the actual mounted configuration of the end product.

Note: Step down chopper power supplies can generate an overvoltage on the output side equivalent to the input voltage

if there is an assembly error on the PC board or if the IC fails. Therefore we strongly recommend the use of a

crowbar or other overvoltage protection circuit in power supply designs.

No. 4472-6/6

This catalog provides information as of July, 1996. Specifications and information herein are subject to change

without notice.

s No products described or contained herein are intended for use in surgical implants, life-support systems, aerospace

equipment, nuclear power control systems, vehicles, disaster/crime-prevention equipment and the like, the failure of

which may directly or indirectly cause injury, death or property loss.

s Anyone purchasing any products described or contained herein for an above-mentioned use shall:

ΠAccept full responsibility and indemnify and defend SANYO ELECTRIC CO., LTD., its affiliates, subsidiaries and

distributors and all their officers and employees, jointly and severally, against any and all claims and litigation and all

damages, cost and expenses associated with such use:

 Not impose any responsibility for any fault or negligence which may be cited in any such claim or litigation on

SANYO ELECTRIC CO., LTD., its affiliates, subsidiaries and distributors or any of their officers and employees

jointly or severally.

s Information (including circuit diagrams and circuit parameters) herein is for example only; it is not guaranteed for

volume production. SANYO believes information herein is accurate and reliable, but no guarantees are made or implied

regarding its use or any infringements of intellectual property rights or other rights of third parties.