Supertex inc. AN-D11 Application Note ±500 Volt Protection Circuit Introduction A ±500V protection circuit for low voltage high impedance measuring instruments is shown in Figure 1. The protection is accomplished by limiting the amount of current going into the measuring instrument. The circuit will protect against destructive high voltages inadvertently connected to the probes (VMEAS) of up to 500VDC of positive and negative polarity. LND150N8 Probe + VMEAS Probe Q1 R1 Q2 V1 V2 Measuring Instrument Figure 1 Circuit Description The circuit consists of two transistors, Q1 and Q2, and one resistor, R. Both Q1 and Q2 are Supertex LND150N8, 500V N-channel depletion mode MOSFETs with gate-to-source ESD protection in a SOT-89 surface mount package. Q1 and Q2 are configured back-to-back as two constant current sources with a nominal value of 1.0mA. Resistor R sets the current limiting value. Figure 2 is a typical low voltage high impedance measurement instrument. Figure 3 is a simplified equivalent circuit showing the protection scheme. + V1 V2 1/4 LM324 V0 = 2(V2 - V1) Figure 2 Under the fault condition, the absolute value of VMEAS is greater than the supply voltage, Q1 limits the current to 1.0mA against large positive voltages and Q2 limits the current to 1.0mA against large negative voltages across VMEAS. For example, if VMEAS is connected to ±500V, Q1 and Q2 will limit the input current to ±1.0mA causing the input voltage to the measurement instrument to clamp to 1.3V above its supply voltage (when R = 600Ω) and 0.7V below ground. Typically the measuring instrument has ESD protection diodes connected from both probes to its power supply and ground. The ESD protection diodes can usually handle 1.0mA continuously. In case there are no ESD diodes provided, external diodes D1, D2, D3, and D4 can be added. VSUP VSUP + 100k VSUP _ Under normal operation, the absolute value of VMEAS is less than the supply voltage of the circuit. Q1 and Q2 will be fully on with a maximum guaranteed RDS of 1.0kΩ. Since the instrument’s input impedance is typically very high, say above 10MΩ, the additional 2.0kΩ series resistance from Q1 and Q2 will not affect measurement accuracy. LND150N8 1k 100k 100k VSUP 100k + 1/4 LM324 D1 V 1 R VSUP 1.0mA D2 - D3 R 1.0mA + VMEAS VSUP VSUP Probe V2 Probe D4 Internal ESD Protection Figure 3 Doc.# DSAN-AN-D11 A040213 Supertex inc. www.supertex.com AN-D11 Calculation for Resistor Value PDISS = 600mW = (constant current value)(max. input voltage) For a current limiting value of ±1.0mA, R can be approximated by the following equation: Figures 4A and 4B are pictures of current due to VMEAS vs. VMEAS voltage of the actual circuit. R1 was chosen to be 1.0kΩ. VGS(OFF) R1 = • ( √ ID / IDSS - 1) I D where, ID = desired constant current value, VGS(OFF) = pinch-off voltage, and IDSS = saturation current at VGS = 0V. Conclusion The high voltage protection circuit is ideal for both bench measurement and handheld measurement instruments. It is simple, reliable and cost effective. It eliminates the possibility of input damage to very sensitive and expensive high impedance devices within the measurement instrument. VGS(OFF) and IDSS are device characteristics and will vary from lot to lot. Actual constant current values are not critical as long as the power dissipation of the LND1 is less than 600mW. Figure 4A Figure 4B Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com) Supertex inc. ©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. Doc.# DSAN-AN-D11 A040213 2 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com