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# Is there any sort of protection for "high" input voltages for the L7815CV?

Associate

I am testing a L7815CV to charge a 12V Battery (I will use this in a solar system with a 20W solar panel where the input voltage might range from 0-22V and Imax=1.3A)

• When supplying an input voltage of 21V to the L7815CV, the L7815CV won't let any current pass.
• When I lower the voltage to below 16V, it starts to allow the current to flow.
• As I increase the input voltage, the L7815CV won't allow the current to flow anymore (in several trials, this voltage trigger where current stops to flow varies from around 17 to 20V)
• It will allow the current to flow again only if I lower the input voltage below 16V.

My questions:

• What is this behavior?
• Is this some sort of protection?
• could this be the internal limitation in power dissipation (Table 1. Absolute maximum ratings in the attached datasheet)? If so, what is this limit in Watts?
• What is the lifetime of such an L7815CV?

PS: temperature of the L7815CV was always below 60 Celsius during the tests.

ST Employee

What is the schematics of your battery charger with the L7815?

Are you using input and output capacitors, what is their type and value and how far are they mounted from the L7815?

Do you use a heat sink on the L7815 and what does it look like resp. what thermal resistance does is provide?

Generally the L7815 needs a voltage difference (dropout voltage) of at least 2V, i.e. you need to provide at least 14V to charge a 12V battery.

But you should keep in mind that it is a linear regulator, which means that it has to burn the entire dropout voltage. With an input voltage of 22V and 1.3A charging current, the resulting power is (22V-12V)*1.3A = 13W, which requires a thick heat sink.

Anyway, it looks like your L7815 is in limiting mode, maybe because your circuit is starting to oscillate due to a lack of capacitors or long wires with a lot of inductance.

The L7815 under most circumstances protects itself, as long as your input voltage is below the maximum value of 35V, so lifetime is usually not an issue. Some exceptions apply, like possible input short-circuits, which can damage the internal power section, so an external protection diode would be necessary (see datasheet, fig. 28). Please read section 6.1. of the datasheet for details of the internal protection.

Regards

/Peter