Undervoltage protection circuit design for DC/DC power modules

In electronic systems terms, power supplies are usually considered to be the heart of the system. This is because reliable operation of the whole electronic system depends on the power supplies’ performance and reliability. As well as this, one should also take the input and output voltage ranges into consideration.

So let’s take the input voltage range of the module: in order to ensure stable and reliable operation of the power supply, one must make it work within the normal range of its rated input voltage. However, due to the diversity of some power supply systems, chances are that the fluctuation range of the input voltage could be quite large.

For example, in systems that use batteries for power, over time the output voltage of the batteries decreases continuously and, when the battery voltage falls and becomes lower than the rated voltage range of the DC/DC, the power supply may try to work under unstable conditions for long periods. Some may even work uninterruptedly in abnormal on and off situations, which may damage the DC/DC power module. In these circumstances, you need to adopt an undervoltage protection circuit when designing the power input system, so that whenever the actual input voltage drops to a certain value, the power supply will be turned off directly.

Refer to the undervoltage protection circuit below:

Principle analysis

When the input voltage drops to a certain set value through the voltage distribution of R1 and R2, the bias voltage of Q2 is lower than the VBE voltage, which leads to shut off of Q2, which subsequently causes the base voltage to rise and the shut off of Q1, so that the input voltage reaching the power supply module becomes 0, ensuring the non-operating state and safety of these modules. In practical applications, one can achieve the shut off of Q2 by adjusting the resistance value of R1 and R2 according to the parameters of selected PNP and NPN transistors.

One instance is when using 9-18 V wide input power modules with a battery power supply, when the battery power voltage drops to 8 V but the power is still there. At this moment, the low input voltage is causing an overcurrent input situation, forcing the internal components to work in a high current condition; thus the thermal shock caused could damage the module. It’s strongly recommended to adopt the undervoltage protection circuit suggested consisting of transistors 8550 and 9013, to shut off the power input when the battery voltage drops to below 8 V. Parameters of the circuit are listed in the following table after actual calculation:

Notes

• Due to the parametric difference of each 9013 VBE (maximum value is 1.4 V), the R1 value is changeable, so as to ensure the bias voltage of Q2 is lower than the VBE voltage when the input voltage is 8 V and realise the undervoltage protection for voltage input under 8 V.
• There will be certain voltage drops when the transistors are working, so there will be around a 0.7 V voltage drop in the actual voltage of input Vin and power modules according to the properties of transistors.

For more information on protection design applications, contact Mornsun Power product distributors such as DLPC (Australia and New Zealand), Fairmont Marketing (Victoria and South Australia) and Fero (New Zealand).