Additional protection of the stabilizer from overheating. Scheme, description

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Additional protection of the stabilizer from overheating

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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When designing stabilizers with current limitation, the calculation of the heat sink of the regulating transistor is carried out for the value of the power released on it in emergency mode. The device proposed by the author of the article will help to reduce this power.

To protect the regulator from overheating and damage during overload or short circuit in the load, it is sometimes not enough to simply limit the output current, especially at relatively high output voltage. Using trigger protection is also not always convenient, as it can cause problems with auto-starting the stabilizer when the overload is removed. For example, on the regulating transistor of a stabilizer with an output voltage of 15 V, with a load current of 2 A and an input voltage of 22 V, 14 W power is released in normal mode, and during a short circuit, even if the current is limited to 1,5 A, this power equal to 33 W.

In such cases, a device will help that turns off the stabilizer for a while in the event of an emergency, and then periodically makes control turns on until the overload disappears. In normal mode, the device does not have any effect on the operation of the stabilizer. This feature can be used to additionally protect the control transistor from overheating in stabilizers with a maximum current limit (this is the main condition), including pulse ones.

Additional protection of the stabilizer from overheating

The additional protection device (see diagram) consists of a rectangular pulse generator with a repetition period of about 2 s on the elements DD 1.1 and DD1.2. node for monitoring the output voltage of the protected stabilizer on the zener diode VD3 and transistor VT1, an electronic switch with an inverter (elements DD1.3. DDI.4) and an output transistor VT2. The device is powered by the simplest parametric stabilizer on the VD1 zener diode. connected to the rectifier filter capacitor. The circuit of the protected stabilizer is shown conditionally as one of the options for connecting a protection device.

In normal operation, the transistor VT1 in the protection device is open and the pulses from the generator do not pass to the transistor VT2, which is closed and does not affect the operation of the stabilizer. When an overload occurs and the output voltage decreases to a value that depends on the stabilization voltage of the Zener diode VD3, the transistor VT1 closes and the generator pulses are fed to the base of the transistor VT2. The duty cycle of the pulses is chosen such that the transistor VT2 is in the open state most of the time (1.5 s), shunting the base circuit of the regulator regulator transistor. Then the transistor VT2 closes for about 0,5 s. the stabilizer turns on and, if during this time the output voltage does not reach the nominal value, it turns off again for 1,5 s. Thus, the power dissipated by the regulating transistor of the stabilizer in emergency mode is reduced by almost four times.

If the next time you turn on the voltage at the output of the stabilizer reaches the nominal value (there is no short circuit or overload), the transistor VT1 opens, the transistor VT2 closes and ceases to affect the operation of the stabilizer.

The R5C2 circuit is designed to slow down the response of the device during the initial power-up. Timing relationships can be changed by selecting resistors and capacitors, it is only necessary to take into account the rise time of the voltage at the output of the stabilizer. Zener diode VD3 should be selected (according to the stabilization voltage) depending on the output voltage of the stabilizer.

This device with minor changes can be used for voltage stabilizers of negative polarity or impulse ones. In particular, a switching power supply was tested with the following parameters: voltage - 5 V, rated load current - 7 A. Closing current - 10 A. The transistor VT2 of the additional protection device shunted the control current of the final stages of the regulating transistor during overload.

Author: V. Andreev, Tolyatti, Samara region.

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