The good thing about being a field applications engineer is that you get to work on many different circuits, often all at the same time. While this is interesting, it also presents problems. Jumping from one circuit to another involves disconnecting a spaghetti of leads and probes, and the chance for something going wrong increases exponentially with the number of wires involved.
It’s often the most basic things that are overlooked. While the probes and leads are checked and double checked to ensure everything is in place, if the voltage on the bench power supply is not adjusted correctly, the damage can be catastrophic, causing hours of rework.
The circuit described in this article helps save the day. Being a field applications engineer also results in a myriad of evaluation boards being collected, each in a state of modification, some of which can be repurposed for personal use. This circuit is based on an overvoltage/reverse voltage protection component, designed to protect downstream electronics from incorrect voltages being applied in automotive circuits.
Such events are caused by the automotive battery being connected the wrong way or a load dump event where the alternator becomes disconnected from the battery, causing a rise in voltage applied to the electronics.
Circuit’s design details
As shown in Figure 1, MAX16126 is a load dump protection controller designed to protect downstream electronics from over-/reverse-voltage faults in automotive circuits. It has an internal charge pump that drives two back-to-back N-channel MOSFETs to provide a low loss forward path if the input voltage is within a certain range, configured using external resistors. If the input voltage goes too high or too low, the drive to the gates of the MOSFETs is removed and the path is blocked, collapsing the supply to the load.
Figure 1 This is how over-/reverse-voltage protection circuit works. Source: Analog Devices Inc.
MAX16127 is similar to MAX16126, but in the case of an overvoltage, it oscillates the MOSFETs to maintain the voltage across the load. If a reverse voltage occurs on the input, an internal 1 MΩ between the GATE and SRC pins of the MAX16126 ensures MOSFETs Q1 and Q2 are held off, so the negative voltage does not reach the output. The MOSFETs are connected in opposing orientations to ensure the body diodes don’t conduct current.
The undervoltage pin, UVSET, is used to configure the minimum trip threshold of the circuit while the overvoltage pin, OVSET, is used to configure the maximum trip threshold. There is also a TERM pin connected via an internal switch to the input pin and this switch is open circuited when the part is in shutdown, so the resistive divider networks on the UVSET and OVSET pins don’t load the input voltage.
In this design, the UVSET pin is tied to the TERM pin, so the MOSFETs are turned on when the device reaches its minimum operating voltage of 3 V. The OVSET pin is connected to a potentiometer, which is adjusted to change the overvoltage trip threshold of the circuit.
To set the trip threshold to the maximum voltage, the potentiometer needs to be adjusted to its minimum value and likewise for the minimum trip threshold the potentiometer is at its maximum value. The IC switches off the MOSFETs when the OVSET pin rises above 1.225 V.
The overvoltage clamping range should be limited to between 5 V and 30 V, so resistors are inserted above and below the potentiometer to set the upper and lower thresholds. There are Zener diodes connected across the UVSET and OVSET pins to limit the voltage of these pins to less than 5.1 V.
Assuming a 47-kΩ resistor is used, the upper and lower resistor values of Figure 1 can be calculated.
To achieve a trip threshold of 30 V, Equation 1 is used:
To achieve a trip threshold of 5 V, Equation 2 is used:
Equating the previous equations gives Equation 3:
So,
From this,
Using preferred values, let R3 = 10 kΩ and R2 = 180 kΩ. This gives an upper limit of 29 V and a lower limit of 5.09 V. This is perfect for a 30 V bench power supply.
Circuit testing
Figure 2 shows the prototype PCB. The trip threshold voltage was adjusted to 12 V and the circuit was tested.
Figure 2 Modified evaluation kit illustrate the circuit testing. Source: Analog Devices Inc.
The lower threshold was measured at 5.06 V and the upper threshold was measured at 28.5 V. With a 10-V input and a 1-A load, the voltage measured between input and output was measured at 19 mV, which aligns with the MOSFET datasheet ON resistance of about 10 mΩ.
Figure 3 shows the response of the circuit when a 10-V step was applied. The yellow trace is the input voltage, and the blue trace shows the output voltage. The trip threshold was set to 12 V, so the input voltage is passed through to the output with very little voltage drop.
Figure 3 A 10-V step is applied to the input of MAX16126. Source: Analog Devices Inc.
The input voltage was increased to 15 V and retested. Figure 4 shows that the output voltage stays at 0 V.
Figure 4 A 15-V step is applied to the input of MAX16126. Source: Analog Devices Inc.
The input voltage was reversed, and a –7 V step was applied to the input, with the results shown in Figure 5.
Figure 5 A –7 V step is applied to the input of MAX16126. Source: Analog Devices Inc.
The negative input voltage was increased to –15 V and reapplied to the input of the circuit. The results are shown in Figure 6.
Figure 6 A –15 V step is applied to the input of MAX16126. Source: Analog Devices Inc.
Caution should be exercised when probing the gate pins of the MOSFETs when the input is taken to a negative voltage. Referring to Figure 1, the body diode of Q1 pulls the two source pins toward VIN, which is at a negative voltage. There is an internal 1 MΩ resistor between the GATE and SRC connections of MAX16126, so when a ground referenced 1 MΩ oscilloscope probe is attached to the gate pins of the MOSFETs, the oscilloscope probe acts like a 1 MΩ pull-up resistor to 0 V.
As the input is pulled negative, a resistive divider is formed between 0 V, the gate voltage, and the source of Q2, which is being pulled negative by the body diode of Q1. When the input voltage is pulled to lower than twice the turn-on voltage of Q2, this MOSFET turns on and the output starts to go negative. Using a higher impedance oscilloscope probe overcomes this problem.
A simple modification to the MAX16126 evaluation kit provides reassuring protection from user-generated load dump events caused by momentary lapses in concentration when testing circuits on the bench. If the components in the evaluation kit are used, the circuit presents a low loss protection circuit that is rated to 90 V with load currents up to 50 A.
Simon Bramble specializes in analog electronics and power. He has spent his career in analog electronics and worked at Maxim and Linear Technology, both now part of Analog Devices Inc.
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