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Important Points in the Design of a Power Supply Using a Floating Type Linear RegulatorTerminal protection for linear regulator ICs

2025.01.28

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When a reverse voltage, an overvoltage or the like is applied to terminals of an IC, the output voltage may not rise, or the IC may be damaged. In cases where the following conditions are conceivable, it is recommended that the terminals be protected appropriately. The methods of terminals protection in these cases are explained in this and subsequent articles. Circuits and operations described in the articles apply to the BA1117 series, and may not be applicable to other linear regulators, so appropriate caution must be exercised.

When the Output Voltage is Higher Than the Input Voltage→Reverse Current Bypass

In a circuit in which the capacitance of the output capacitor is large, when charge remains in the output capacitor even after the input power supply has been turned off, and when the speed with which the input power supply turns off is extremely fast, an inverted state may occur in which the output voltage is higher than the input voltage. In such cases, a reverse current flows from the output to the input via a parasitic element within the IC. The parasitic element operation is not intentional, and therefore there is the possibility of element degradation or destruction.
As a countermeasure, either a reverse current bypass diode (D1) may be connected externally such that reverse current does not pass through the IC (left diagram below), or a reverse current blocking diode (D1) may be inserted on the input side (right diagram below).

Reverse current bypass diode/Reverse current blocking diode

However, when a method is used in which the input line is open and the IC is turned off (diagram below), the reverse current value is due only to the IC bias current and is minuscule, and consequently degradation or destruction of the parasitic element will not occur, and so a bypass diode is unnecessary.

Case in which input is open

The bypass diode must be turned on (conducting) in advance of the parasitic element within the IC. In the BA1117 series linear regulators, the turn-on voltage for internal parasitic elements is about 0.7 V, and so a diode with a forward voltage VF lower than this is needed. The diode should have a rated reverse voltage larger than the input/output voltage difference being used (80% derating or less), and a rated forward current larger than the reverse current value (50% derating or less). Given these conditions, a Schottky barrier diode is recommended as the bypass diode; but because many Schottky barrier diodes have large reverse currents, a device with a small reverse current value should be selected.

When the Output Load is Inductive → Output reverse voltage protection

When the output load is inductive, energy stored in the inductive load is discharged to ground at the moment the output voltage turns off. There is an electrostatic destruction prevention diode in the IC between the VOUT terminal and the ADJ terminal; the current that flows in the diode at this time can cause the IC to be destroyed. In order to prevent this, a Schottky barrier diode (D1 in the diagram below) is connected in parallel with the electrostatic destruction prevention diode.

Current paths for an inductive load (when output turns off)

Representative inductive loads include motors, solenoids, and relays. When the load is a motor, a back emf in the motor causes current to flow in internal diodes, and so an external protective diode is necessary. One inductive load that tends to be overlooked is the inductance of a long wire that connects the output terminal of an IC to a load. An oscilloscope should be used to observe the voltage waveform to ensure that a transient reverse voltage is not occurring when the output turns off.

When There is a Possibility of Polarity-Reversed Connection of the Input Power Supply → Input Reverse Voltage Protection

When connecting a power supply to an input terminal, if the polarity of the power supply is reversed due to inattention or for some other reason, a current flows in the internal electrostatic destruction prevention diode between the VIN terminal and the ADJ terminal, possibly resulting in destruction of the IC and the resistor R2 (left diagram below).

Current paths with input reverse-connected/Reverse connection countermeasure 1

The simplest way to deal with reverse connections is to insert either a Schottky barrier diode or a rectifier diode in series with the input power supply line (right diagram above, Reverse connection countermeasure 1). When the connection is correct, there is a voltage drop equal to the forward voltage VF of the diode, and so a power loss equal to VF×IO occurs, so that this method is not suited to battery-driven circuits. Because VF is lower for a Schottky barrier diode than for a rectifier diode, using the former as the inserted diode results in somewhat smaller losses. This loss in the diode causes heat generation, and so a device with an adequate allowable loss margin should be selected. When reverse connection occurs, a reverse current flows in the diode, but this current is negligible.

In the circuit of the reverse connection countermeasure 2 (left diagram below), a method is shown in which a diode is connected in parallel with the power supply. Because it must be turned on sooner (at a lower voltage than) the electrostatic destruction prevention diode within the IC, a Schottky barrier diode with a low VF is used. When the connection is correct, the operation is the same as when no diode is present.

Reverse connection countermeasure 2/Reverse connection countermeasure 3

Upon a reverse connection, all the current from the power supply keeps flowing through this diode, so that a large amount of heat is generated, and if the current capacitance of the input power supply is large, destruction may result. This type of circuit presupposes that either protection is provided against accidental reverse connection for short lengths of time, or that an overcurrent protection circuit is installed in the input power supply. If there is a need to further improve the safety of this protection circuit, a fuse or polyswitch may be added in series with the input power supply line, as in reverse connection countermeasure 3 (right diagram above). A fuse requires maintenance, but is capable of more reliable circuit protection.

Reverse connection countermeasure 4 (left diagram below) is a method in which a P-channel MOSFET is inserted in series with the input power supply line. The diode which is shown between the drain and source of the MOSFET is a body diode (parasitic element) of the MOSFET. In the case of a correct connection, the P-channel MOSFET is turned on, and so the voltage drop across the MOSFET is the product of the on-resistance of the MOSFET and the output current IO; this voltage drop is much smaller than the voltage drop that results from diode insertion in the reverse connection countermeasure 1, and so the power loss is smaller. Should a reverse connection occur, the MOSFET is not turned on, so no current flows. If the MOSFET gate-source voltage (with derating considered) exceeds the rated voltage, voltage dividing resistors should be added between the gate and source as in reverse connection countermeasure 5 (right diagram below) to lower the gate-source voltage.

Reverse connection countermeasure 4/Reverse connection countermeasure 5

When Hot Plugging Possible → Hot plugging countermeasure

When wiring is connected to the input of an IC with the supply-side (input) power supply in the on-state, a pulse voltage (surge) occurs in the IC input line due to the inductance component of the wiring and the metal contact of the connecting plug. If this surge voltage exceeds the absolute maximum rating for the IC, destruction of the IC may result. A TVS (Transient Voltage Suppressor) diode (D1 in the diagram below) can be used to provide protection by absorbing such surges so that a surge voltage does not enter the IC input terminal.

Hot plugging countermeasures

When a Load Exists between Disparate Power Supplies → Reverse Current Bypass

The diagram below shows an example in which a load is present between different power supplies. In such a case, the voltages of the power supplies do not rise and fall with the same timing, and so current flows through the load from one power output terminal to the other. In such cases, a reverse-voltage state occurs in which the voltage at the output of the IC is higher than the input voltage, and therefore reverse current bypass diodes (D1、D2) are needed. This is the same as the approach explained in “When the Output Voltage is Higher than the Input Voltage → Reverse Current Bypass”.

Current paths between different power supplies and diodes insertion

When There are Both Positive and Negative Power Supplies (Dual Supply)

The following is an example of a positive-negative power supply that uses the BA1117 for the positive voltage. In such a positive-negative power supply, the speed of voltage rise is different for the two power supplies, and when a load is present between the positive and negative power supplies, the power supply that has risen first pulls current from the other output through the load so that a reverse voltage occurs at the output. In order to prevent damage to the IC and failure of the output voltage to rise, low- VF Schottky diodes (D1、D2) must be connected between the outputs and GND.

Insertion of positive-negative power supply diodes, and current paths when the negative power supply regulator voltage has risen first

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