Supertex inc. AN-D17 Application Note High Voltage Off-Line Linear Regulator by Jimes Lei, Applications Engineering Manager Introduction to minimize the loading on batteries, especially when the vehicles are not in use for long periods of time. For example, only a few microamperes are needed for powering memory ICs. In such situations the quiescent current of the regulator should be within a few microamperes. There are many applications for small, linear voltage regulators that operate from high input voltages. They are ideally suited for powering CMOS ICs, small analog circuits, and other loads requiring low current. These circuits can be used in several applications requiring power directly from the utility line. They can also be used for applications which either have very wide input voltage variations or environments with high voltage spikes; for example, telecommunications, automotive, and avionics. This application note discusses several circuits which will benefit these applications. The high voltage protected, 5.0V linear regulator shown in Figure 1 meets all of the above requirements. It is very simple, compact and inexpensive. The high operating voltage and high transient voltage protection are achieved by using Supertex part #LND150N8 in conjunction with a 5.0V linear regulator, Ricoh part #RH5RA50AA. Direct off-line applications require operation at 120VAC to 240VAC which corresponds to maximum peak voltages of ±340V. Applications in telecommunications, automotive, and avionics require immunity against very fast, high voltage transients. In telecommunications, the high voltage transients are caused by lightning or spurious radiations. In automotive and avionics they are caused by inductive loads such as ignition coils and electrical motors. International Standards Organization specification ISO/TR7637, for electrical interference by conduction and coupling in automobiles, shows that transients up to -300V and +120V can be generated due to various inductive loads. Circuit Description The LND150N8 is a 500V, N-channel, depletion-mode MOSFET. It has a maximum RDS(ON) of 1.0KΩ, VGS(OFF) of -1.0 to -3.0V, and an IDSS of 1.0 to 3.0mA. The RH5RA50AA is a 5.0V ±2.5% voltage regulator with a maximum quiescent current of 1.0µamp. Both these parts are available in the SOT-89 (TO-243AA) surface mount package. The high voltage input, HVIN, is connected to the anode of diode D. The cathode of the diode is connected to the drain of the LND1. The diode is used as protection against negative transient voltages and as a half-wave rectifier for off-line application. The LND1 is connected in the source follower configuration, with its gate connected to the output, VOUT, and its source to the input ofthe 5.0V regulator, VIN. Capacitors C1, C2 and C3 are bypass capacitors. C3 is required when HVIN is negative, such as during the negative half cycle of an AC line, or negative transients. The proper value of C3 is chosen based on the worst case duration and duty cycle of the negative pulses on HVIN. In addition to the ability to withstand high voltages, many circuits used for the above mentioned applications also require low quiescent current. The low quiescent current is required to minimize power dissipation in these linear regulators. Many telecommunication applications require very low quiescent current because there are limitations to the allowable current that can be drawn from the telephone lines. Automotive and avionics applications require low quiescent current Figure 1: High Voltage Universal Off-Line Linear Regulator HVIN IN4005 D LND150N8 C3 C2 0.01μF Supertex inc. + VGS RH5RA50AA VOUT = 5.0V C1 0.01μF RL ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 1 AN-D17 HVIN, VIN and VOUT are at 0V before a voltage is applied to HVIN. The LND1 is turned on when its gate-to-source voltage, VGS = 0V. Once a voltage is applied to HVIN, current will flow through the diode and the “normally on” channel of the LND1 charging capacitor C2. The voltage across C2 is connected to VIN. As VIN starts to increase, VOUT will also continue to increase until it reaches its regulated voltage of 5.0V. The LND1 is configured as a source follower with its gate connected to a fixed 5.0V value (nominal). The voltage on the source, VIN, will follow the voltage on its gate, minus VGS. VIN = VOUT -VGS where VGS is the voltage required to supply the input current IIN. If 500VDC is applied on HVIN, VOUT will remain at 5.0V and VIN should be between 6.0 to 8.0V, since VGS(OFF) of LND150N8 is guaranteed to be -1.0 to -3.0V. The actual observed value was 6.26V. The dropout voltage, (VIN -VOUT), for the 5.0V regulator with a 1.0mA load is rated as 30mV. To maintain regulation, VIN must be equal to or greater than 5.03V. As IIN increases, VIN decreases and thereby increases the gate-to-source voltage on the LND1 to meet the IIN requirement. The transfer characteristics of the LND1 give a good indication of VGS vs. IIN. DC Operation The LND1 increases the maximum operating voltage range from 13.5 to 500VDC. In order for the output to maintain regulation, the voltage difference (VIN -VOUT), must be greater than the regulator’s specified dropout voltage of 30mV at 1.0mA load current. The measurements are shown below: HVIN IIN VIN VOUT Conditions 10 to 500V 770nA 6.26V 5.02V No load 10 to 500V 503µA 5.56V 5.02V 10KΩ 10 to 500V 1.0mA 5.30V 5.02V 5.0KΩ Since the LND150N8 is connected in a source follower configuration, the value of VIN can be estimated as shown in Figure 2. Figure 2: VIN Calculation HVIN D ID G VOUT S Advantages of the LND1 The important parameters of the LND1 are its 500V breakdown voltage, 1.5pF output capacitance and 1.0MΩ dynamic output impedance. Supertex utilizes a proprietary design and fabrication process to achieve very flat output characteristics which gives this device its very high dynamic impedance, rO. The RH5RA50AA has an absolute maximum input voltage rating of 13.5V. The highbreakdown voltage of the LND1 extends the maximum input operating voltage range from 13.5V to 500V. The low output capacitance and high dynamic impedance prevent the input voltage of the RH5RA50AA from exceeding its absolute maximum value of 13.5V when very fast high voltage transients are present. The ripple rejection ratio is also improved by several orders of magnitude. C2 VIN ID = IDSS • (1 - VGS / VGS(OFF) )2 VGS = VOUT - VIN VIN = VOUT - VGS(OFF) • (1 - √ID / IDSS ) High Voltage Transient Protection Positive and negative transient voltages were applied on HVIN. The positive transient voltages are blocked by the LND1 and the negative transient voltages are blocked by the 1N4005 diode, which has a 600V PIV rating. LND1 improves the performance of the 5.0V linear regulator in the areas listed below. Observations and measurements were taken under three different loading conditions: no load, 10KΩ, and 5.0KΩ. a) DC operation extended from 13.5 to 500V b) High voltage transient protection c) Greatly improved ripple rejection ratio d) Eliminates power-up transients Supertex inc. ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 2 AN-D17 Figure 3: Positive Transient Test Condition HVIN Figure 5: Estimate VIN Increase Due to Transients VIN C2 0.01μF ID HVIN LND1 VOUT REG C1 0.01μF CDS 1.5pF Ro 5kΩ C2 0.01µF 50msec 310V HVIN rO = AC resistance, typically 1.0MΩ (almost no effect on VIN) ≈ 10V 500nsec VIN tR = 10nsec ID = CDS dv/dt = 1.5pF • (300V/10ns) = 45mA I • dt (45mA) • (10ns) Figure 3 shows the test conditions used for simulating transient voltages. Positive 300V pulses with a pulse width of 500nsec, a rise time of 10nsec, and a duty cycle of 1.0% are superimposed on the 10VDC line of HVIN. Figures 4a and 4b are waveforms showing HVIN, VIN and VOUT. ∆VIN = ∆VIN = 45mVPEAK The low drain-to-source capacitance, CDS = COSS - CRSS = 1.5pF, and high dynamic output impedance, rO = 1.0MΩ, of the LND1 inherently give the LND1 excellent frequency response. The LND1 configured as a source follower will effectively protect high voltage transients on HVIN from affecting VIN. The only paths for transient voltages to get into VIN are through the 1.5pF CDS or 1.0MΩ rO. Any transient voltages that pass through will be further attenuated by C2. The increase in VIN caused by the transient voltage can be estimated with the equivalent circuit shown in Figure 5. Negative 300V pulses with a pulse width of 500nsec, a rise time of 10nsec, and a duty cycle of 1.0% are superimposed on the 10VDC line of HVIN. The 1N4005 diode is reverse biased and blocks the negative voltage. Figures 6a and 6b are waveforms showing HVIN, VIN, and VOUT. Figure 4a: HVIN and VIN C2 = 0.01µF The LND1 with the 1N4005 effectively protects the input of the 5.0V regulator from positive and negative transient voltages. Theoretical and measured values indicated VIN will never exceed its maximum rating of 13.5V. Figure 4b: HVIN and VOUT HVIN = 310V HVIN = 310V VIN = 5.4V VOUT = 5.1V 10V 10V Supertex inc. ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 3 AN-D17 Figure 6a: HVIN and VIN Figure 6b: HVIN and VOUT HVIN = 10V HVIN = 10V VIN = 5.4V VOUT = 5.1V -300V -300V Ripple Rejection Ratio The ripple rejection ratio, RR, demonstrates the LND150N8’s capability of filtering AC ripple on the input of HVIN. A 4.0VP-P, 1.0MHz sinusoidal signal was applied to the 5.0V regulator with and without the LND1. Figure 7 shows the test conditions. The amount of AC attenuation due to the LND1 can be estimated by the equivalent circuit and equations shown in Figure 8. Figure 8: Ripple Rejection Calculation HVIN Figure 7: Ripple Rejection Test Conditions HVIN VIN = HVIN = 9.0VDC + 2.0sin2πftV f = 1.0MHz VREG C2 0.01μF VIN VIN VREG VOUT C1 0.01μF RL VOUT C 0.01μF RL VIN = CDS 1.5pF 2sin2πftv f = 1.0MHz - CDS CDS + C2 C2 0.01μF VIN • HVIN 1.5pF VIN = • (4.0P-P ) 1.5pF + 0.01µF VIN = 600µVP-P The ripple rejection ratio was improved by a factor of 1000. Such a high ripple rejection ratio is particularly useful for off-line applications. A typical 240VAC off-line application is shown in Figure 9a. Measured results are as follows: Peak-to-peak output AC voltage, VOUT RR = 20log 4.0V Figure 9a: 240VAC Off-Line 5.0V Regulator IN4005 VOUT with LND1 VOUT without LND1 Conditions 1.3mV, RR = -70dB 2.90V, RR = -2.8dB No load 1.3mV, RR = -70dB 2.90V, RR = -2.8dB 10KΩ 1.3mV, RR = -70dB 2.90V, RR = -2.8dB 5.0KΩ Supertex inc. + VDRAIN C3 0.04μF + HVIN - 240VAC C2 0.01μF VREG ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 4 VOUT C1 0.01μF RL 5k AN-D17 Figure 9b: VDRAIN and VOUT While there was a large overshoot voltage without the LND1, no overshoots were observed in the circuit employing the LND1. Loads prone to damage by overshoots can be effectively protected by using the LND1. VDRAIN = 340V Conclusion 50V VOUT Figure 9b shows the voltage waveforms at the drain, VDRAIN, of the LND1 and the AC voltage at VOUT. There were 290V of AC ripple observed on VDRAIN with less than 2.0mV of ripples on VOUT. C3 is a high voltage holding capacitor. In order to minimize size and cost, more often than not it is desirable to select C3 to be as small as possible. The high ripple rejection ratio helps in achieving a small size of C3 because it allows for large AC input voltage with negligible AC output voltage. Power-Up Transient Suppression The circuits shown in Figures 10a and 10b are powered up from 0 to 10V in 100nsec. This test demonstrates the stability of the circuit, the amount of overshoot voltage on VOUT, and the amount of time required for the output to settle. Large overshoot voltages on VOUT may damage sensitive loads, such as CMOS circuits. The high voltage protected, low power, 5.0V linear regulator in Figure 1 is a robust, compact, cost effective regulator. It can operate up to 500VDC, protect against ±500V transients, and has a maximum quiescent current of 1.0µA. The electrical characteristics of the LND1 allow for the 500V operation and protection. Some examples are proximity controlled light switches, street lamp control, fax machines, modems, and power supplies for CMOS ICs in automotive, avionics and a variety of applications. Other Application Ideas The circuit in Figure 1 can be easily modified for higher current capability. The LND1 can be replaced by the Supertex DN2540N5, which is a 400V, 150mA depletion mode MOSFET in a TO-220 package. In case the current is low, and the worst case power dissipation for the DN25 is below 1Watt, the TO-92 version (part #DN2540N3) can be used to save space and cost. Figure 11 utilizes an op-amp and an enhancement-mode MOSFET for a much higher output current capability. Figure 12 is an off-line street lamp control where VSENSE is the input voltage from a light sensing device. The test results were: With LND1 Without LND1 Conditions VPEAK tr VPEAK tr 0.0V 50µsec 7.6V 1.0µsec No load 0.0V 60µsec 7.0V 1.0µsec 10KΩ 0.0V 80µsec 6.9V 1.0µsec 5.0KΩ Supertex inc. ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 5 AN-D17 Figure 10b: Power Up Response without LND1 Figure 10a: Power Up Response with LND1 HVIN LND1 VIN C2 0.01μF HVIN 10V 0V VOUT 5V 0V VOUT REG C1 0.01μF VIN VOUT REG C1 0.01μF RL VIN 10V 0V tr = 100nsec VPEAK RL tr = 100nsec 5V 0V VOUT tr tr Figure 11: High Output Current Linear Regulator HVIN C3 D LND150N3 C2 5.0V RH5RA50AA C1 R2 VSET + R1 - VN0340N5 Max406 VOUT = VSET C4 Figure 12: Off-Line Street Lamp Controller D2 D1 C3 LND150 + VIN - 120VAC C2 D3 5.0V RH5RA50AA VSENSE C1 VN0640N5 Max406 + - R1 Lamp R3 R2 C4 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. ©2012 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. 011812 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com 6