Supertex inc. AN-D10 Application Note Off-Line Compact Universal Linear Regulator Introduction An off-line compact universal linear regulator is shown in Figure 1. The regulating device is the Supertex LND150N3. The LND1 is a 500V N-channel depletion mode MOSFET with gate-to-source ESD protection. The regulated voltage, VOUT, is an ideal supply for CMOS ICs and a variety of other circuits that require low current. the 120VAC input line. D5 does not conduct during normal operation. Calculations for Component Values C1 is a 0.1µF 200V capacitor, chosen to minimize ripple on the 170VDC which would affect the regulated output voltage. The minimum value of C1 is calculated as follows: Circuit Description The 120V AC input voltage is rectified by a full bridge, consisting of diodes D1, D2, D3, and D4. A small filter or smoothing capacitor, C1, is used to hold the rectified voltage to approximately +170VDC. The unregulated 170VDC is connected to the drain of the LND1. The LND1 and trim pot R1 are configured as a 1.0mA constant current source. The 1.0mA constant current flows through R2 which is a 5.1kΩ resistor to ground. A constant voltage drop of 5.1V is developed across R2. VOUT is taken as the voltage across R2 and is used to supply, for example, a simple CMOS timer circuit. Capacitor C2 is a low voltage bypass capacitor to supply any peak current required by the CMOS timer circuit during switching transitions. D5 is a 5.6V Zener diode used to clamp transient voltages that may occur during powering up VIN = Asin2πft; A = 170V, f = 60Hz I = C1 dv ; I = 1.0mA dt dv = ΔV = A - VOUT - (ID(ON) • RDS(ON) ); dt = Δt = C1 ≥ I Δt = 1.0mA • ΔV • 1 2f 1 2(60Hz) 170V - 5.1V - (10mA) • (1KΩ) C1 ≥ 0.054μF The LND1 can maintain a virtually constant current over a wide input voltage range. Large ripple voltages on the drain of the LND1 will have very little effect on the output current. The device can also withstand transient voltages up to 500V. 1500pF + 120VAC/ 240VAC D4 VIN _ D2 D6 D1 D3 C1 0.1µF 3M 0.1mA LND150N3 Potter & Brumfield KUMP 11A-18-240 3M 1500pF VDD D1, D2, D3, D4, D6, D7: IN4005 VDD R1 10k + D5 5.6V _ CK R2 5.1k T Reset C2 1.0µF MC14030 14-BIT Counter VN0650N3 Load 100k CMOS Timer Figure 1. Linear Regulator Doc.# DSAN-AN-D10 A090313 Supertex inc. www.supertex.com AN-D10 VDRAIN and VOUT vs Time 170V VDRAIN 0V 5V VOUT 0V Figure 2. Input/ Output Waveforms The value of the constant current source is a function of R1, VGS(OFF), and IDSS where VGS(OFF) and IDSS are characteristics of the device. R1 is a variable resistor adjusted for 1.0mA and is approximated by: R1 = Where, ID VGS(OFF) ID on VOUT over a period of time. C2 can be calculated as follows: • ( √ ID / IDSS - 1 ) IOUT = output current dt = required time duration of IOUT dVOUT = acceptable change in VOUT For example, a 10.0mA output peak current for a duration of 1.0µsec with a maximum VOUT drop of 100mV will require a C2 value of 1.0mA(1.0µsec/100mV) = 0.1µF or greater. C2 is chosen to be 1.0µF. = desired constant current value, VGS(OFF) = pinch-off voltage, and IDSS C2 = IOUT(dt/dVOUT) where, = saturation current at VGS = 0 V. Figure 2 is an oscilloscope picture showing the actual voltage waveforms on the drain of the LND1 and VOUT. VGS(OFF) and IDSS may vary from lot to lot. The range of adjustment for R1 is calculated for operation over the range of LND1 values for VGS(OFF) and IDSS. Symbol Parameter Min Max VGS(OFF) Pinch-off voltage -1.0V -3.0V IDSS Saturation current -1.0mA -3.0mA Figure 3 is an output characteristic showing the regulation of the circuit over a wide range of input voltage. VOUT 5.0V For the above values, R1 is calculated to be from 0 to 1.3kΩ. A 10kΩ trim pot is chosen for R1. Since the constant current is adjusted to 1.0mA, R2 is chosen to be 5.1kΩ to obtain a VOUT of 5.1VDC. The value of C2 is selected to supply the peak current required by the load Doc.# DSAN-AN-D10 A090313 6.8V VIN Figure 3. VOUT vs VIN 2 Supertex inc. www.supertex.com AN-D10 For a 10V source, R2 can be replaced with a 10KΩ resistor. Applications requiring multiple voltage references can be generated using a string of resistors as shown in Figure 4. 500V since the LND1 breakdown voltage is guaranteed to be greater than 500V. A simple, low cost, transient protection (e.g., MOV) will protect the circuit from virtually anything, other than a direct lightning strike. The constant current can easily be changed by readjusting R1 for the desired current. However, the power dissipation on the LND1 should be taken into consideration. PD for the LND1 in the TO-92 package should not exceed: Regulation can also be achieved with AC or DC voltages from 6.8 to 240V with no modifications of the circuit. This allows manufacture of one model of equipment for worldwide usage without any voltage setting tappings. IDS (VIN - VOUT ) = 600mW. Conclusion Alternative Applications The Supertex LND1 can be configured as a simple, constant current source to create an economical compact off-line, low current regulated, voltage supply for powering CMOS ICs and other low current loads. The need for transformers can be eliminated. Universality The universality of the linear regulator can benefit a variety of industrial or consumer applications as it can be used from a very wide range of input voltages, anywhere in the world. Input voltages can be up to 450V for linear regulation. Protection is afforded for line voltage transients up to + VIN _ 120VAC/ 240VAC D4 D2 D1 C D3 1.0mA LND1 5.0VDC 1kΩ 4.0VDC 1kΩ 3.0VDC 1kΩ 2.0VDC 1kΩ 1.0VDC 1kΩ 0VDC Figure 4. Multiple Voltage References 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-D10 A090313 3 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com