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High Efficiency Linear Regulators Application Note 32 March 1989 High Effi ciency Linear Regulators Jim Williams Introduction Linear voltage regulators continue to enjoy widespread use switching supply output. Figure 1 shows such an arrange- despite the increasing popularity of switching approaches. ment. The main output (“A”) is stabilized by feedback to Linear regulators are easily implemented, and have much the switching regulator. Usually, this output supplies most better noise and drift characteristics than switchers. Ad- of the power taken from the circuit. Because of this, the ditionally, they do not radiate RF, function with standard amount of energy in the transformer is relatively unaf- magnetics, are easily frequency compensated, and have fected by power demands at the “B” and “C” outputs. fast response. Their largest disadvantage is ineffi ciency. This results in relatively constant “B” and “C” regulator Excess energy is dissipated as heat. This elegantly sim- input voltages. Judicious design allows the regulators to plistic regulation mechanism pays dearly in terms of lost run at or near their dropout voltage, regardless of loading power. Because of this, linear regulators are associated or switcher input voltage. Low dropout regulators thus with excessive dissipation, ineffi ciency, high operating save considerable power and dissipation. temperatures and large heat sinks. While linears cannot L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear compete with switchers in these areas they can achieve Technology Corporation. All other trademarks are the property of their respective owners. signifi cantly better results than generally supposed. New $V components and some design techniques permit reten- +VIN tion of linear regulator’s advantages while improving VREG “C” effi ciency. One way towards improved effi ciency is to minimize the input-to-output voltage across the regulator. The smaller $V this term is, the lower the power loss. The minimum input/ VREG “B” output voltage required to support regulation is referred to as the “dropout voltage.” Various design techniques and technologies offer different performance capabilities. Appendix A, “Achieving Low Dropout,” compares some approaches. Conventional three terminal linear regulators “A” have a 3V dropout, while newer devices feature 1.5V drop- out (see Appendix B, “A Low Dropout Regulator Family”) SWITCHING at 7.5A, decreasing to 0.05V at 100μA. REGULATOR Regulation from Stable Inputs AN32 F01 Lower dropout voltage results in signifi cant power sav- ings where input voltage is relatively constant. This is Figure 1. Typical Switching Supply Arrangement Showing Linear normally the case where a linear regulator post-regulates a Post-Regulators an32f AN32-1 Application Note 32 Regulation from Unstable Input—AC Line Derived Case Unfortunately, not all applications furnish a stable input the output voltage. The 15V output comparison still favors voltage. One of the most common and important situations the low dropout regulator, although effi ciency benefi t is is also one of the most diffi cult. Figure 2 diagrams a classic somewhat reduced. Figure 4 derives resultant regulator situation where the linear regulator is driven from the AC power dissipation from Figure 3’s data. These plots show line via a step-down transformer. A 90VAC (brownout) to that the LT1086 requires less heat sink area to maintain 140VAC (high line) line swing causes the regulator to see the same die temperature as the LM317. a proportionate input voltage change. Figure 3 details ef- Both curves show the deleterious effects of poorly con- fi ciency under these conditions for standard (LM317) and trolled input voltages. The low dropout device clearly cuts low dropout (LT®1086) type devices. The LT1086’s lower losses, but input voltage variation degrades obtainable dropout improves effi ciency. This is particularly evident effi ciency. at 5V output, where dropout is a signifi cant percentage of 15 14 13 12 LM317 VO = 15V, 1A 11 10 LT1086 VO = 15V, 1A 9 VIN—MOVES WITH 8 LM317 VO = 5V, 1A AC LINE VOLTAGE 7 LINEAR 6 V VOLTAGE OUT 5 REGULATOR 90 TO 140 4 AN32 F02 3 VAC LINE IN WATTS POWER DISSIPATION 2 LT1086 VO = 5V, 1A 1 + 0 80 90 100 110120 130 140 AC LINE VOLTAGE AN32 F04 Figure 2. Typical AC Line Driven Linear Regulator Figure 4. Power Dissipation for Different Regulators vs AC Line Voltage. Rectifi er Diode Losses are not Included 120 110 LT1086 WITH SCR PRE-REGULATOR 100 V = 15V, 1A V = 16.5V VOUT = 15V, 1A OUT 90 IN V = 18V 80 IN VIN = 6.5V LM317 70 LT1086 60 VIN = 8V VIN = 25.6V VIN = 28V 50 LT1086 WITH VIN = 10.1V EFFICIENCY (%) 40 SCR PRE-REGULATOR VOUT = 5V, 1A LT1086 LM317 VIN = 12.4V V = 5V, 1A 30 OUT 20 DATA DOES NOT INCLUDE RECTIFIER 10 DIODE LOSSES. SCR PRE-REGULATOR LM317 DROPOUT = 3V INCLUDES SCR LOSSES LT1086 DROPOUT = 1.5V 0 80 90 100 110 120 130 140 150 AC LINE VOLTAGE AN32 F03 Figure 3. Effi ciency vs AC Line Voltage for LT1086 and LM317 Regulators an32f AN32-2 Application Note 32 SCR Pre-Regulator Figure 5 shows a way to eliminate regulator input variations, SCR and a path from the main transformer to L1 (Trace D) even with wide AC line swings. This circuit, combined with occurs. The resultant current fl ow (Trace E), limited by L1, a low dropout regulator, provides high effi ciency while charges the 4700μF capacitor. When the transformer out- retaining all the linear regulators desirable characteristics. put drops low enough the SCR commutates and charging This design servo controls the fi ring point of the SCRs to ceases. On the next half-cycle the process repeats, except stabilize the LT1086 input voltage. A1 compares a portion that the alternate SCR does the work (Traces F and G are of the LT1086’s input voltage to the LT1004 reference. The the individual SCR currents). The loop phase modulates amplifi ed difference voltage is applied to C1B’s negative the SCR’s fi ring point to maintain a constant LT1086 input input. C1B compares this to a line synchronous ramp voltage. A1’s 1μF capacitor compensates the loop and its (Trace B, Figure 6) derived by C1A from the transformers output 10kΩ-diode network ensures start-up. The three rectifi ed secondary (Trace A is the “sync” point in the terminal regulator’s current limit protects the circuit from fi gure). C1B’s pulse output (Trace C) fi res the appropriate overloads. L1 500μH ≈16.5V TO 17V 15V L2 LT1086 OUT 1N4148 + 1A + 121Ω* 20VAC 4700μF 158k* 20μF AT 115VIN (TRIM) 90VAC TO 1k 140VAC 1.37k* 20VAC AT 115VIN 12k* 1N4002 1N4002 “SYNC” +V 1N4002 10k ≈30V TO ALL + 200k “+V” POINTS 1N4148 + C1A 22μF 750Ω 1/2 LT1018 – 0.1μF +V 0.001 2.4k + C1B 1/2 LT1018 1N4148 + – A1 LT1006 10k 10k 10k – +V 1μF +V L1 = PULSE ENGINEERING, INC. #PE-50503 LT1004 L2 = TRIAD F-271U (TYPICAL) 1.2V * = 1% FILM RESISTOR AN32 F05 = G.E. C-106B Figure 5. SCR Pre-Regulator an32f AN32-3 Application Note 32 This circuit has a dramatic impact on LT1086 effi ciency DC Input Pre-Regulator versus AC line swing*. Referring back to Figure 3, the Figure 8a’s circuit is useful where the input is DC, such data shows good effi ciency with no change for 90VAC as an unregulated (or regulated) supply or a battery. This to 140VAC input variations. This circuit’s slow switching circuit is designed for low losses at high currents. The preserves the linear regulators low noise. Figure 7 shows LT1083 functions in conventional fashion, supplying a slight 120Hz residue with no wideband components. regulated output at 7.5A capacity. The remaining com- *The transformer used in a pre-regulator can signifi cantly infl uence overall ponents form a switched-mode dissipation regulator. effi ciency. One way to evaluate power consumption is to measure the actual power taken from the 115VAC line. See Appendix C, “Measuring This regulator maintains the LT1083 input just above the Power Consumption.” dropout voltage under all conditions. When the LT1083 input (Trace A, Figure 9) decays far enough, C1A goes high, A = 50V/DIV B = 10V/DIV C = 50V/DIV D = 20V/DIV A = 5mV/DIV E = 10A/DIV (AC-COUPLED) F = 10A/DIV G = 10A/DIV HORIZ = 5ms/DIV AN32 F06 HORIZ = 5ms/DIV AN32 F07 Figure 6. Waveforms for the SCR Pre-Regulator Figure 7. Output Noise for the SCR Pre-Regulated Circuit Q1 L2 P50N05E 335μH ≈6.75V 12V DS 5V IN LT1083-5 OUT (7V TO 20V) + + 1N966 + + 7.5A 10μF 330Ω 47μF 16V 1000μF 10μF L1 MBR1060 820μH 100Ω 1N4148 5.1k 56k* (TRIM) 1N4148 270k ≈30V 12V + 22μF 100k D – Q2 C1B VN2222 0.01 S 1/2 LT1018 + – 1N4148 C1A 10k 1/2 LT1018 12k* + 20k 12k 12V 10k AN32 F08a 12V LT1004 47pF 1.2V 2k 1M L1 = PULSE ENGINEERING, INC. #PE-52630 L2 = PULSE ENGINEERING, INC. #PE-51518 P50N05E = MOTOROLA Figure 8a. Pre-Regulated Low Dropout Regulator an32f AN32-4 Application Note 32 FROM LT1083 V L2 + OUT 120 470Ω 10μF 12V 1N4148 1k V ≈ 1.8V 4N28 REF – 10k AN32 F08b TO REMAINING 1/2 LT1018 CIRCUITRY + 20k 12k 12V 1N4148 47pF 1M Figure 8b. Differential Sensing for the Pre-Regulator Allows Variable Outputs 100 VIN = 12V 90 80 70 • • • 60 • 50 A = 100mV/DIV (AC-COUPLED) 40 EFFICIENCY (%) 30 B = 20V/DIV 20 10 C = 20V/DIV 0 0 132 4 5 6 7 8 D = 4A/DIV OUTPUT CURRENT (A) AN32 F10 HORIZ = 100μs/DIV AN32 F09 Figure 9. Pre-Regulator Waveforms Figure 10.Effi ciency vs Output for Figure 8a allowing Q1’s gate (Trace B) to rise.
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