Hardware Protection – OverVoltage and OverCurrent


Power processing is one of the most important aspects on electronic design. The power is unique for a typical system because it gives the system life. Before starting to make the project for a power supply we need to analyze some aspects: Which kind of radio/electromagnetic interference is the device going to face ? What about maintenance requirements? And finally which environment conditions (temperature, humidity, vibrations) will the device be exposed to?

1. Power Supply Parameters

Three major power supply (Figure 1) architectures are defined for any engineer:
1.Linear Regulators.
2.Pulse width modulated switching (PWM).
3.High resonant technology switching.

Power Supply Concept

Figure 1: Powe Supply Concept

Some important parameters can described in the followig texts:

Vin(low), Vin(high) – minimum and maximum allowed input voltages, hence input voltage range.
Iin(max) – maximum average input current.

Vout(min), Vout(max) - minimum and maximum allowed output voltages, hence output voltage range.
Vout(abs) – maximum allowed output voltage this limit is set by load specs. Iout(rated), Iout(min) – maximum and minimum output current provided to the load. Isc – short circuit output current.

2. Protection

Startup Evolution of Power Supply Parameters

Figure 2: Startup Evolution of Power Supply Parameters

When some of these inputs/outputs parameters are not correlated with their optimal values or just when some external event disturbs the system, like dropout, surge, transients, ripple voltages, the entire system is vulnerable and need to be protected. Everyone knows that is better preventing than curing. The solution for this is permanent monitoring and auto controlling. In most of the cases an electronic circuit is build for controlling a specific load (it’s the output terminal). The load can be a sensor, a resistive circuit or even a microcontroller (Figure 2).

Current Protection:
Depending on power line, load and control circuit layout, two basic architectures are available: low-side switch and high-side switch (figure 3 e 4).

Typical Connection for Current Sensing

Figure 3: Typical Connection for Current Sensing

Current Sensing for the Two Basic Architectures

Figure 4: Current Sensing for the Two Basic Architectures

Voltage Protection:

What if a disturb like the one reported in 5.A is handled by the protection shown in 5.b? Picture 5.c shows how the protection circuit behaves.

OverVoltage Basic Protection

Figure 5: Over Voltage Basic Protection

  • Diode P: Clamps input voltage to maximum voltage VCL.
  • Resistance RS: Limits the dissipated energy in the protection component without compromising the clamping function.
  • Diode D: When reverse-biased, it clamps input voltage protecting from negative impulsive over voltage.
  • Capacitor C: Its role is to make sure that the voltage at the terminals of the electronic unit is greater than or equal to Vcc(min) while the starter circuit is active.

4. I/O Control

Sometimes an embedded system is designed to control loads which run on high current or voltage. Most of the processors run at 3.3V or 5V, so the outputs are not able for switching on/off some loads connected to 12V, 24V or even more. The solution is to use some very fast turn on/off transistors, MOS technology, which are often named simply “ MOSFET switches” (Figure 6 and 7).

Basic Switch Control

Figure 6: Basic Switch control

High-Side/Low-Side Switch

Figure 7: Hig-Side/Low-Side Switch

Depending by the application the switches can be of two types: low side or high side. Let’s present below some typical schemes for protection with these kinds of switches.

I/O Current Limitation:

Current limitations

Figure 8: I/O Current limitation

Rs is the sense resistor and Vref is the threshold voltage above which the system starts to correct itself. More precisely, when the voltage across Rs raises above Vref, the output of the operational amplifier increases and sets the gate of MOS transistor to reduce voltage, in this way reducing also the output current through the load.

I/O Thermal Shutdown:

Thermal Shutdown

Figure 9: I/O Thermal shutdown

The thermal detection is done with a heat sensing element located near the switch. Sometimes the thermal protection is incorporated into the switch box and interacts directly over the gate of the MOS transistor. The scope is to turn completely off the circuit before something bad happens.

I/O Current Protection Against Output Short Circuit:

I/O Current Protection on Load Short Circuit

Figure 10: I/O Current Protection Against Output Short Circuit

When the load is short circuited, an over current flows in the circuit. The detection must be very quickly determined to protect the rest of the circuit. Depending by the type of the protection logic, the switch-off state can be made permanent or just temporary.

I/O Open Load Protection:

Open Load Protection

Figure 11: I/O Open load Protection

A recurrent problem in embedded systems is load disconnection. This can be done accidentally or because the load is mechanically disconnected. The voltage across drain-source of this low-side switch is permanently compared with Vooref. Open load is indicated/detected when the switch is off and the continuity is maintained across Vpwr, Load and Output. A microcontroller can be signaled about this fault and can turn off the power.

Based on text written by RarCod


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