Supertex inc. AN-D30 Off-Line 5.0V Output Non-Isolated Linear Regulator Introduction There are many applications that call for a non-isolated, low current DC power supply operating directly from the AC line. A switchmode power supply would be far too complex and expensive, whereas a simple 60Hz step down transformer would be cost effective but physically too large. Examples of such applications include battery chargers, proximity switches, television stand-by supplies, and internal supplies for switchmode power supplies. When line isolation is not necessary, the circuit presented in this application note provides a solution that is both very cost effective and compact. The circuit presented in this note is a two-stage linear regulator capable of providing 5.0V at 50mA, but can be easily modified for other output voltages. The first stage handles the majority of voltage drop and power dissipation, allowing the second stage to employ standard low-voltage, lowpower linear IC regulators. The Supertex DN2535N5 high voltage depletion-mode MOSFET is used as the first stage pass transistor. For an isolated output, or an output with significantly higher output currents, please refer to Supertex application notes AN-H13, AN-H21, AN-H22, AN-H23, and AN-H24 which discuss the Supertex HV91XX series of switchmode PWM controller ICs. Application Note Circuit Description The circuit of Figure 1 provides a regulated 5.0V output at 50mA directly from a 120VAC input. Detailed descriptions will be given for the three different sections: Input rectifier, pre-regulator, and output regulator. SPICE simulation, lab measurements, and power dissipation are also addressed. Caution The circuit described in this application note does NOT provide galvanic isolation. When operated from an AC line, potentially lethal voltages can be present within the circuit. Adequate means of protecting the end user from such voltages must be provided by the circuit developer. Design Requirements The circuit in Figure 1 was developed to meet the design requirements listed in the table below. Many other output voltages and currents can be achieved simply by changing component values, without requiring any alterations in circuit topology. Input Output 5.0VDC ±4% 120VAC 0 to 50mA 50mVP-P ripple voltage Pre-Regulator 120VAC C2 150pF D1-D4 1N4001 Output-Regulator U1 120VAC Q1 DN2535N5 Z1 9.1V C1 10µF LM78L05 R1 100kΩ Load C3 1.0µF Figure 1: 5V @ 50mA Non-Isolated Power Supply Doc.# DSAN-AN-D30 A040313 Supertex inc. www.supertex.com AN-D30 Circuit Description Component Function Notes Value Rating D1-D4 Rectifies AC line current - 1N4004 (120VAC) 1N4005 (240VAC) 200V (120 VAC) 350V (240VAC) 100mA Q1 Preregulator pass transistor Use appropriate heat sink DN2535N5 (120VAC) DN2540N5 (240VAC) 350V (120VAC) 400V (240VAC) 150mA Z1 Sets preregulator output voltage Other zener voltagesmay be used for other output voltages 1N757 (9.1V) 1/10W R1 Provides bias for Q1 and Z1 - 100kΩ 1/16W C1 Stores energy for use when AC < 9.1V - 10µF 15V C2 Prevents high frequency oscillations Locate close to Q1 150pF 200V (120VAC) 350V (240VAC) U1 Provides output regulation Other devices may be used for other output voltages LM78L05 (5.0V) 50mA C3 Reduces transients at output - 1.0µF 10V (or greater than output voltage) Section I: Input Rectifier Section II: Pre-Regulator Figure 2 shows the line rectification circuit. Diodes D1 through D4 are selected to handle the maximum input voltage and load current. Recommended rectifiers are 1N4004’s for 120V AC line or 1N4005’s for 240V AC line. Pre-Regulator C2 150pF 120VAC Q1 DN2535N5 D1-D4 1N4001 Z1 9.1V 120VAC Figure 3 shows the preregulator, providing high voltage input to low voltage output using the Supertex DN2535N5. The preregulator must supply an input voltage to the output regulator within a range as determined below: Power dissipation in the bridge rectifier is calculated to be: Prect = 2 • (ILOAD + IBIAS) • VF VSOURCE(MIN) = minimum specified input voltage for the ouput regulator ILOAD = load current (50mA) IBIAS = bias current for Z1 and U1 (5.5mA) VF = rectifier forward voltage drop (0.7V) This yields 78mW dissipation in the bridge rectifier. Doc.# DSAN-AN-D30 A040313 C1 10µF Figure 3: Pre-Regulator Figure 2: Input Rectifier where: R1 100KΩ VSOURCE(MAX) = VOUT + PREG(MAX) ILOAD(MAX) or maximum specified input voltage, whichever is less 2 Supertex inc. www.supertex.com AN-D30 where: VSOURCE(MAX) = voltage at source of Q1 = input voltage to output regulator VAC(PK) = peak line voltage and the 1.4V accounts for rectifier drops For the given circuit, this works out to be 241µS. The minimum value of C1 given an allowable maximum voltage droop is: ILOAD(MAX) = maximum anticipated load current Preg(MAX)= power dissipation rating for the output regulator VZ1 = Zener voltage VOUT = final output voltage For the LM78L05, the minimum input voltage is specified as 7.0V. The maximum input voltage is specified as 30V. However, the 700mW@25OC power rating of the LM78L05 limits the maximum input voltage to 17.6V. C ≥ tOFF • ILOAD + IBIAS VZ1 - VSOURCE(MIN) where: ILOAD = load current Q1 acts as a source follower where the source voltage follows the gate voltage minus the gate-source voltage (VGS): IBIAS = bias current for Z1 and U1 VZ1 = zener voltage VSOURCE(MIN) = minimum input voltage for U1 VSOURCE = VGATE - VGS VGS increases with increasing drain current, thus with a fixed gate voltage, the source voltage will drop with increasing load current. For design purposes, VGS for the transistor under saturation and cut-off conditions (0V and VGS(OFF), respectively) can be used. These values can be readily obtained from the transistor data sheet. For the DN2535N5, VGS(OFF) can be a maximum of -3.5v. This works out to 6.4µF. The next highest standard value, 10µF, was selected. Q1 must recharge C1 immediately after the rectified AC again rises above 9.1V. For this reason, the transistor’s saturation current (IDSS) must be greater than the load and bias currents. The DN2535N5’s IDSS is 150mA minimum, providing more than enough current to recharge C1. Zener Z1 sets the gate voltage and should be selected to provide a source voltage within the range determined above, taking into account the variances of VGS with load. A zener voltage of 9.1V will result in a source voltage of 9.1 to 12.6V under all load conditions. The power dissipation for Q1 can be calculated from the voltage drop across it times the current through it: PQ1 = (VDRAIN - VSOURCE) • (ILOAD + IBIAS) Bias current for the Zener is determined by VGS/R2. Using 100kΩ for R2, the bias current can vary between 0 and 35µA, although the actual bias current will be less than 35µA since the transistor is not operated at complete cut-off. Vdrain is the rectified 120VACrms line minus two diode drops (1.4V) for a drain voltage of approximately 118.6Vrms. VSOURCE has been previously determined to be in the range of 9.1 to 12.6 volts. Therefore, the maximum voltage across Q1 is about 109.5V. Current through Q1 is the sum of the load current plus the bias currents for Z1 and U1, or 50mA + 5.5mA. This yields a power dissipation in Q1 of about 6.1W. An adequate heat sink must be provided for Q1 to dissipate this power. Power dissipation of the other components in the preregulator are insignificant. Storage capacitor C1 must store enough energy to supply the load for periods when the rectified AC voltage is below 9.1 volts. The duration of this period is: 1 (VZ1 - 1.4V) tOFF = arcsin πf (VAC(PK) ) where: f = line frequency Doc.# DSAN-AN-D30 A040313 Capacitor C2 across the drain-source of Q1 is needed to avoid possible high frequency oscillations due to parasitic inductances. A value of 150pF is sufficient. 3 Supertex inc. www.supertex.com AN-D30 Section III: Output Regulation The preregulator output voltage, for both simulation and actual results, are close to the design voltage of 10 volts. The output regulator consists of an LM78L05 linear regulator and C3. U1 provides an output of 5.0V±4% at 50mA. C3 serves to reduce output transients. The output regulator, LM78L05, provided a solid 5.0V output. Rejection of the 2.0V input ripple was excellent, showing no discernible output ripple. Experimental results showed about 20mVPP of noise at the output. 2nd Stage Regulator LM78L05 U1 Load C3 1µF 240VAC Variation A slight variation of this circuit is to power a 25mA load from a 240VAC line. Adjustment of the above circuit, replacing the 100Ω load resistor with a 200W and increasing the line voltage to 240VAC. Storage capacitor C1 may be halved since current has been halved. Overall power dissipation is about the same since the load current was halved and the line voltage doubled. The voltage rating of Q1 could remain at 350V, since the peak voltage of a 240VAC line is 336V. However, this only provides a 4% safety margin which does not allow for variations in line voltage. The DN2540N5 with a 400V rating is recommended, providing a 19% safety margin. Recommended rectifiers for the bridge rectifier are 1N4005’s. Figure 2: Input Rectifier Power dissipation for the output regulator can be calculated as: PU1 = PBIAS + PREG = (VIN x IBIAS) + (VOUT - VREG) x IOUT = 10V x 5.5mA + (10V - 5V) x 50mA = 0.31W Measurements The circuit of Figure 1 was constructed and measurements were taken. Overall, the circuit performed close to what simulations predicted. The preregulator output voltage is very close to simulation and calculations. Simulations predicted that VSOURCE would drop 2.0V. Experimental results confirmed this. VDRAIN (DC, 50V/div) VSOURCE (DC, 2.0V/div) VOUT (AC, 50mV/div) Figure 5: Experimental results showing VDRAIN, VSOURCE, and VOUT (top to bottom). Doc.# DSAN-AN-D30 A040313 4 Supertex inc. www.supertex.com AN-D30 Design Verification Based on SPICE simulations, the circuit of Figure 1 should yield waveforms similar to those in Figure 6. 200V 0V 15V 0V V(RECT) V(OUT) • V(REG) 10ms 20ms 30ms 34ms Time Figure 6: SPICE simulation results for 5.0V, 50mA load 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-D30 A040313 5 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com