The benefit of being a subject purposes engineer is that you just get to work on many various circuits, usually all on the similar time. Whereas that is fascinating, it additionally presents issues. Leaping from one circuit to a different entails disconnecting a spaghetti of leads and probes, and the possibility for one thing going unsuitable will increase exponentially with the variety of wires concerned.
It’s usually essentially the most staple items which might be neglected. Whereas the probes and leads are checked and double checked to make sure every little thing is in place, if the voltage on the bench energy provide is just not adjusted accurately, the harm will be catastrophic, inflicting hours of rework.
The circuit described on this article helps save the day. Being a subject purposes engineer additionally leads to a myriad of analysis boards being collected, every in a state of modification, a few of which will be repurposed for private use. This circuit is predicated on an overvoltage/reverse voltage safety element, designed to guard downstream electronics from incorrect voltages being utilized in automotive circuits.
Such occasions are attributable to the automotive battery being linked the unsuitable method or a load dump occasion the place the alternator turns into disconnected from the battery, inflicting an increase in voltage utilized to the electronics.
Circuit’s design particulars
As proven in Determine 1, MAX16126 is a load dump safety controller designed to guard downstream electronics from over-/reverse-voltage faults in automotive circuits. It has an inner cost pump that drives two back-to-back N-channel MOSFETs to supply a low loss ahead path if the enter voltage is inside a sure vary, configured utilizing exterior resistors. If the enter voltage goes too excessive or too low, the drive to the gates of the MOSFETs is eliminated and the trail is blocked, collapsing the provision to the load.
Determine 1 That is how over-/reverse-voltage safety circuit works. Supply: Analog Gadgets Inc.
MAX16127 is just like MAX16126, however within the case of an overvoltage, it oscillates the MOSFETs to take care of the voltage throughout the load. If a reverse voltage happens on the enter, an inner 1 MΩ between the GATE and SRC pins of the MAX16126 ensures MOSFETs Q1 and Q2 are held off, so the unfavorable voltage doesn’t attain the output. The MOSFETs are linked in opposing orientations to make sure the physique diodes don’t conduct present.
The undervoltage pin, UVSET, is used to configure the minimal journey threshold of the circuit whereas the overvoltage pin, OVSET, is used to configure the utmost journey threshold. There’s additionally a TERM pin linked by way of an inner change to the enter pin and this change is open circuited when the half is in shutdown, so the resistive divider networks on the UVSET and OVSET pins don’t load the enter voltage.
On this design, the UVSET pin is tied to the TERM pin, so the MOSFETs are turned on when the machine reaches its minimal working voltage of three V. The OVSET pin is linked to a potentiometer, which is adjusted to alter the overvoltage journey threshold of the circuit.
To set the journey threshold to the utmost voltage, the potentiometer must be adjusted to its minimal worth and likewise for the minimal journey threshold the potentiometer is at its most worth. The IC switches off the MOSFETs when the OVSET pin rises above 1.225 V.
The overvoltage clamping vary must be restricted to between 5 V and 30 V, so resistors are inserted above and beneath the potentiometer to set the higher and decrease thresholds. There are Zener diodes linked throughout the UVSET and OVSET pins to restrict the voltage of those pins to lower than 5.1 V.
Assuming a 47-kΩ resistor is used, the higher and decrease resistor values of Determine 1 will be calculated.
To realize a visit threshold of 30 V, Equation 1 is used:
To realize a visit threshold of 5 V, Equation 2 is used:
Equating the earlier equations offers Equation 3:
So,
From this,
Utilizing most well-liked values, let R3 = 10 kΩ and R2 = 180 kΩ. This offers an higher restrict of 29 V and a decrease restrict of 5.09 V. That is good for a 30 V bench energy provide.
Circuit testing
Determine 2 exhibits the prototype PCB. The journey threshold voltage was adjusted to 12 V and the circuit was examined.
Determine 2 Modified analysis package illustrate the circuit testing. Supply: Analog Gadgets Inc.
The decrease threshold was measured at 5.06 V and the higher threshold was measured at 28.5 V. With a 10-V enter and a 1-A load, the voltage measured between enter and output was measured at 19 mV, which aligns with the MOSFET datasheet ON resistance of about 10 mΩ.
Determine 3 exhibits the response of the circuit when a 10-V step was utilized. The yellow hint is the enter voltage, and the blue hint exhibits the output voltage. The journey threshold was set to 12 V, so the enter voltage is handed by way of to the output with little or no voltage drop.
Determine 3 A ten-V step is utilized to the enter of MAX16126. Supply: Analog Gadgets Inc.
The enter voltage was elevated to fifteen V and retested. Determine 4 exhibits that the output voltage stays at 0 V.
Determine 4 A 15-V step is utilized to the enter of MAX16126. Supply: Analog Gadgets Inc.
The enter voltage was reversed, and a –7 V step was utilized to the enter, with the outcomes proven in Determine 5.
Determine 5 A –7 V step is utilized to the enter of MAX16126. Supply: Analog Gadgets Inc.
The unfavorable enter voltage was elevated to –15 V and reapplied to the enter of the circuit. The outcomes are proven in Determine 6.
Determine 6 A –15 V step is utilized to the enter of MAX16126. Supply: Analog Gadgets Inc.
Warning must be exercised when probing the gate pins of the MOSFETs when the enter is taken to a unfavorable voltage. Referring to Determine 1, the physique diode of Q1 pulls the 2 supply pins towards VIN, which is at a unfavorable voltage. There’s an inner 1 MΩ resistor between the GATE and SRC connections of MAX16126, so when a floor referenced 1 MΩ oscilloscope probe is connected to the gate pins of the MOSFETs, the oscilloscope probe acts like a 1 MΩ pull-up resistor to 0 V.
Because the enter is pulled unfavorable, a resistive divider is fashioned between 0 V, the gate voltage, and the supply of Q2, which is being pulled unfavorable by the physique diode of Q1. When the enter voltage is pulled to decrease than twice the turn-on voltage of Q2, this MOSFET activates and the output begins to go unfavorable. Utilizing a better impedance oscilloscope probe overcomes this downside.
A easy modification to the MAX16126 analysis package offers reassuring safety from user-generated load dump occasions attributable to momentary lapses in focus when testing circuits on the bench. If the elements within the analysis package are used, the circuit presents a low loss safety circuit that’s rated to 90 V with load currents as much as 50 A.
Simon Bramble makes a speciality of analog electronics and energy. He has spent his profession in analog electronics and labored at Maxim and Linear Know-how, each now a part of Analog Gadgets Inc.
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