DESIGN FEATURES L DC/DC Converter, Capacitor Charger Takes Inputs from 4.75V to 400V by Robert Milliken and Peter Liu Introduction High voltage power supplies and capacitor chargers are readily found in a number of applications, including professional photoflashes, security control systems, pulsed radar systems, satellite communication systems, and explosive detonators. The LT3751 makes it possible for a designer to meet the demanding requirements of these applications, including high reliability, relatively low cost, safe operation, minimal board space and high performance. The LT3751 is a general purpose flyback controller that can be used as either a voltage regulator or as a capacitor charger. The LT3751 operates in boundary-mode, between continuous conduction mode and discontinuous conduction mode. Boundary-mode operation allows for a relatively small transformer and an overall reduced PCB footprint. Boundary-mode also reduces large signal stability issues that could arise from using voltagemode or PWM techniques. Regulation is achieved with a new dual, overlapping modulation technique using both damage. When used as a regulator, the LT3751’s feedback loop is internally compensated to ensure stability. The LT3751 is available in two packages, either a 20-pin exposed pad QFN or a 20-lead exposed pad TSSOP. 2V/DIV GND 250ns/DIV Figure 1. Gate driver waveform in a typical application peak primary current modulation and duty-cycle modulation, drastically reducing audible transformer noise. The LT3751 features many safety and reliability functions, including two sets of undervoltage lockouts (UVLO), two sets of overvoltage lockouts (OVLO), no-load operation, over-temperature lockout (OTLO), internal Zener clamps on all high voltage pins, and a selectable 5.6V or 10.5V internal gate driver voltage clamp (no external components needed). The LT3751 also adds a start-up/shortcircuit protection circuit to protect against transformer or external FET New Gate Driver with Internal Clamp Requires No External Components There are four main concerns when using a gate driver: output current drive capability, peak output voltage, power consumption and propagation delay. The LT3751 is equipped with a 1.5A push-pull main driver, enough to drive +80nC gates. An auxiliary 0.5A PMOS pull-up only driver is also integrated into the LT3751 and is used in parallel with the main driver for VCC voltages of 8V and below. This PMOS driver allows for rail-to-rail operation. Above 8V, the PMOS driver must be deactivated by tying its drain to VCC. Most discrete FETs have a VGS limit of 20V. Driving the FET higher than 20V could cause a short in the internal gate oxide, causing permanent DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VTRANS 10V TO 24V T1* 1:10 + OFF ON C1 10µF TO µP R1, 154k VTRANS R2, 475k VCC C2 2.2µF s5 R6 40.2k C3 680µF RVTRANS CHARGE CLAMP RDCM R7 18.2k VCC R8 40.2k LT3751 DONE RVOUT HVGATE LVGATE CSP FAULT UVLO1 OVLO1 CSN UVLO2 FB R9 D2 • C4 100µF • VOUT 50V TO 450V + C5 0.47µF VOUT 100V/DIV GND VCC M1 R5 6mΩ 1W ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED D1,D2: VISHAY MURS260 M1: IRF3710Z T1: WURTH 750310349 OVLO2 GND RBG D1 4.7nF Y RATED * LIMIT OUTPUT POWER TO 40W FOR 65°C T1 MAX AMBIENT OPERATION IIN(AVG) 2A/DIV 0 VIN = 24V COUT = 100µF 20ms/DIV Figure 3. Isolated high voltage capacitor charger charging waveform Figure 2. Isolated high voltage capacitor charger from 10V to 24V input Linear Technology Magazine • March 2009 L DESIGN FEATURES DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VTRANS 10V TO 24V T1** 1:10 + OFF ON C1 10µF TO µP R1, 154k VTRANS R6 40.2k C3 680µF R2, 475k VCC C2 2.2µF s5 RVTRANS CHARGE CLAMP RDCM R7 18.2k VCC R8 40.2k LT3751 DONE RVOUT HVGATE LVGATE CSP FAULT UVLO1 OVLO1 CSN VCC D1 D2 • C4 100µF • VOUT 400V + C5 0.47µF C4: CDE 380LX101M500J042 C5: TDK CKG57NX7R2J474M D1,D2: VISHAY MURS260 M1: IRF3710Z T1: WURTH 750310349 M1 * USE TWO SERIES 1206, 1% RESISTORS FOR R10 R10: 249k s2 R5 6mΩ 1W ** LIMIT OUTPUT POWER TO 40W FOR 65°C T1 MAX R10* AMBIENT OPERATION 499k UVLO2 OVLO2 ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED FB C6 10nF GND RBG R9 787Ω R11 1.54k Figure 4. A 10V to 24V input, 400V regulated power supply damage. To alleviate this issue, the LT3751 has an internal, selectable 5.6V or 10.5V gate driver clamp. No external components are needed, not even a capacitor. Simply tie the CLAMP pin to ground for 10.5V operation or tie to VCC for 5.6V operation. Figure 1 shows the gate driver clamping at 10.5V with a VCC voltage of 24V. Not only does the internal clamp protect the FET from damage, it also reduces the amount of energy injected into the gate. This increases overall efficiency and reduces power consumption in the gate driver circuit. The gate driver overshoot is very minimal, as seen in Figure 1. Placing the external FET closer to the LT3751 HVGATE pin reduces overshoot. High Voltage, Isolated Capacitor Charger from 10V to 24V Input The LT3751 can be configured as a fully isolated stand-alone capacitor charger using a new differential discontinuous-conduction-mode (DCM) comparator—used to sense the boundary-mode condition—and a new differential output voltage (VOUT) comparator. The differential operation of the DCM comparator and VOUT comparator allow the LT3751 to accurately operate from high voltage input supplies of greater than 400V. Likewise, the LT3751’s DCM comparator and VOUT comparator can work with input supplies down to 4.75V. This accommodates an unmatched range of power sources. Figure 2 shows a high voltage capacitor charger driven from an input supply ranging from 10V to 24V. Only five resistors are needed to operate the LT3751 as a capacitor charger. The output voltage trip point can be continuously adjusted from 50V to 450V by adjusting R9 given by: 0.98 • N R9 = • R8 VOUT(TRIP) + VDIODE The LT3751 stops charging the output capacitor once the programmed output voltage trip point (VOUT(TRIP)) is reached. The charge cycle is repeated by toggling the CHARGE pin. The maximum charge/discharge rate in 90 402 EFFICIENCY VDRAIN 20V/DIV GND GND IPRIMARY 5A/DIV 0 IPRIMARY 5A/DIV 0 10µs/DIV a. Switching waveform for IOUT = 100mA 80 401 75 VOUT (V) VDRAIN 20V/DIV EFFICIENCY (%) 85 LOAD REGULATION 70 400 65 10µs/DIV b. Switching waveform for IOUT = 10mA 60 0 20 40 60 80 LOAD CURRENT (mA) 399 100 c. Efficiency and load regulation Figure 5. High voltage regulator performance 10 Linear Technology Magazine • March 2009 DESIGN FEATURES L the output capacitor is limited by the temperature rise in the transformer. Limiting the transformer surface temperature in Figure 2 to 65°C with no air flow requires the average output power to be ≤40W given by: VOUT AC RIPPLE 10V/DIV IIN(AVG) 20mA/DIV 0 PAVG = 1 C • FREQUENCY • 2 OUT 2VOUT(TRIP) • VRIPPLE – VR2IPPLE ( 2s/DIV ) Figure 6. The LT3751 protecting the output during a no-load condition ≤ 40 W where VOUT(TRIP) is the output trip voltage, VRIPPLE is the ripple voltage on the output node, and frequency is the charge/discharge frequency. Two techniques are used to increase the available output power: increase the airflow across the transformer, or increase the size of the transformer itself. Figure 3 shows the charging waveform and average input current for a 100µF output capacitor charged to 400V in less than 100ms (R9 = 976Ω). For output voltages higher than 450V, the transformer in Figure 2 must be replaced with one having higher primary inductance and a higher turns ratio. Consult the LT3751 data sheet for proper transformer design procedures. High Voltage Regulated Power Supply from 10V to 24V Input The LT3751 can also be used to convert a low voltage supply to a much higher voltage. Placing a resistor divider from the output node to the FB pin and ground causes the LT3751 to operate as a voltage regulator. Figure 4 shows a 400V regulated power supply operating from an input supply range of 10V to 24V. The LT3751 uses a regulation control scheme that drastically reduces audible noise in the transformer and the input and output ceramic bulk capacitors. This is achieved by using an internal 26kHz clock to synchronize the primary winding switch cycles. Within the clock period, the LT3751 modulates both the peak primary current and the number of switching cycles. Figures 5a and 5b show heavy-load and light-load waveforms, respectively, while Figure 5c shows efficiency over most of the operating range for the application in Figure 4. The clock forces at least one switch cycle every period which would overcharge the output capacitor during a no-load condition. The LT3751 handles no-load conditions and protects against over-charging the output node. Figure 6 shows the LT3751 protecting during a no-load condition. Resistors can be added to RVOUT and RBG to add a second layer of protection, or they can be omitted to reduce component count by tying RVOUT and RBG to ground. The trip level for the VOUT comparator is typically set 20% higher than the nominal regulation voltage. If the resistor divider were to fail, the VOUT comparator would disable switching when the output climbed to 20% above nominal. DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY T1*** D1 1:3 F1, 1A C3 100µF 450V + R7 88.7k + 7.5k OFF ON VCC C1 10µF R2**, 9M R3, 154k 4.7nF Y RATED RVOUT FAULT R1** 1.5M VCC LT3751 DONE TO µP VTRANS RVTRANS CHARGE RDCM CLAMP R4, 475k HVGATE LVGATE • R8 137k ×3 R9 66.5k R10* 208k R11 14.7k + 17.4k R13,20Ω VCC CSP UVLO2 CSN OVLO2 GND RBG R5 1.11k FB + C5 0.47µF 630V ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED M1 FQB4N80 UVLO1 OVLO1 C4 220µF 550V • VOUT 500V R12 68mΩ 1/4W C4: HITACHI PS22L221MSBPF C5: TDK CKG57NX7R2J474M T1: COILCRAFT HA4060-AL D1,D2: VISHAY US1M F1: BUSSMANN PCB-1-R * USE THREE SERIES 1206, 0.1% RESISTORS FOR R6 & R10 R6: 249k ×2 + 127k R10: 66.5k ×2 + 75k ** USE TWO SERIES 1206, 1% RESISTORS FOR R1 & R2 R1: 750k ×2 R2: 4.53M ×2 530 1000 520 850 VOUT,TRIP 700 510 500 490 100 CHARGE TIME 200 300 INPUT VOLTAGE (V) CHARGE TIME (ms) VCC 10V TO 24V C2 2.2µF 630V s5 R6* 625k D2 VOUT,TRIP (V) VTRANS 100V TO 400V DC 550 400 400 Figure 8. Isolated capacitor charger VOUT(TRIP) and charge time with respect to input voltage *** OUTPUT POWER LIMITED TO 20W FOR 65°C T1 AMBIENT OPERATION Figure 7. A 100V to 400V input, 500V output, isolated capacitor charger Linear Technology Magazine • March 2009 11 L DESIGN FEATURES DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VTRANS 100V TO 400V DC T1*** D1 1:3 F1, 1A + R7 95.3k C3 100µF OFF ON VCC 10V TO 24V C1 10µF TO µP R6* 615k C2 2.2µF s5 RVTRANS CHARGE RDCM CLAMP VCC LT3751 VTRANS R2**, 9M R3, 154k VCC R4, 475k • C4 100µF • VOUT 400V ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED + R8* 411k C5 0.47µF C4: CDE 380LX101M500J042 C5: TDK CKG57NX7R2J474M T1: COILCRAFT HA4060-AL D1,D2: VISHAY US1M F1: BUSSMANN PCB-1-R R9 66.5k RVOUT * USE THREE SERIES 1206, 1% RESISTORS FOR R6 & R8 R6: 205k ×3 R8: 137k ×3 DONE FAULT HVGATE LVGATE R1**, 1.5M D2 R13,20Ω VCC ** USE TWO SERIES 1206, 1% RESISTORS FOR R1, R2 & R11 R1: 750k ×2 R2: 4.53M ×2 R11: 249k ×2 M1 FQB4N80 UVLO1 OVLO1 CSP UVLO2 OVLO2 GND RBG *** OUTPUT POWER LIMITED TO 20W FOR 65°C T1 AMBIENT OPERATION R10 68mΩ ¼W CSN R11** 499k FB C6 10nF R12 1.54k Figure 9. A 100V to 400V input, 400V output, capacitor charger and voltage regulator Note that the FB pin of the LT3751 can also be used for a capacitor charger. The LT3751 operates as a capacitor charger until the FB pin reaches 1.225V, after which the LT3751 operates as a voltage regulator. This keeps the capacitor topped-off until the application needs to use its energy. The output resistor divider forms a leakage path from the output capacitor to ground. When the output voltage droops, the LT3751 feedback circuit will keep the capacitor topped- off with small, low current bursts of charge as shown in Figure 6. High Input Supply Voltage, Isolated Capacitor Charger As mentioned above, the LT3751 differential DCM and VOUT comparators allow the part to accurately work from very high input supply voltages. An offline capacitor charger, shown in Figure 7, can operate with DC input voltages from 100V to 400V. The transformer provides galvanic isolation from 90 398 OUTPUT VOLTAGE (V) EFFICIENCY (%) 80 70 60 VIN = 100V VIN = 250V VIN = 400V 50 40 0 25 50 75 397 396 IOUT = 10mA IOUT = 25mA IOUT = 50mA 395 100 OUTPUT CURRENT (mA) 200 300 INPUT VOLTAGE (V) a. Overall efficiency b. Line regulation 400 the input supply to output node—no additional magnetics required. Input voltages greater than 80V require the use of resistor dividers on the DCM and VOUT comparators (charger mode only). The accuracy of the VOUT trip threshold is heightened by increasing current IQ through R10 and R11; however, the ratio of R6/R7 should closely match R10/R11 with tolerances approaching 0.1%. A trick is to use resistor arrays to yield the desired ratio. Achieving 0.1% ratio accuracy is not difficult and can reduce the overall cost compared to using individual 0.1% surface mount resistors. Note that the absolute value of the individual resistors is not critical, only the ratio of R6/R7 and R10/R11. The DCM comparator is less critical and can tolerate resistance variations greater than 1%. The 100V to 400VDC input capacitor charger has an overall VOUT(TRIP) accuracy of better than 6% over the entire operating range using 0.1% resistor dividers. Figure 8 shows a typical performance for VOUT(TRIP) and charge time for the circuit in Figure 7. Figure 10. High voltage input and output regulator performance 12 Linear Technology Magazine • March 2009 DESIGN FEATURES L ISOLATION BOUNDARY DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY D2 R2, 10Ω T1 • Npb VTRANS 100V TO 200V DC F1, 2A + R1 49.9k 1/2W M1 C2 1µF D1 1 1 TO µP ALL RESISTORS ARE 0805,1% RESISTORS UNLESS OTHERWISE NOTED C7: 330FK400M22X38 D1: 12V ZENER D2: MURS140 D3: P6kE200A D4, D5: STTH112A D6: BAT54 D7: BAS516 M1: IRF830 M2: STB11NM60FD T1: TDK SRW24LQ-UxxH015 (Np:Ns:Npb:Nsb=1:2:0.08:0.08) U1: PS2801-1 U2: LT4430 R9, 2.7M VTRANS R10, 4.3M R11, 84.5k VCC R12, 442k C4 1µF 250V 1 s2 R3 210k RVTRANS CHARGE CLAMP OFF ON 1 C1 100pF 1 C3 22µF 350V 1 s2 VCC RDCM RVOUT FAULT HVGATE LVGATE CSP UVLO1 OVLO1 4.7nF Y RATED R8 2.49k 1 2 • M2 VCC C7 400µF 330V C5 0.01µF 630V R6 40mΩ 1/4W R15 221k R16, 1k C9 3.3µF R13 5.11Ω R7 475Ω C10 0.47µF D7 U1 2 U2 D6 2 1 R14 249k 2 2 D4 1 GND RBG + C6 0.1µF 630V Nsb CSN FB • Ns 2 R5, 210k UVLO2 OVLO2 Np D5 R4, 105k LT3751 DONE • D3 VOUT 282V 225mA VIN COMP C8 22nF LT4430 OC R17 3.16k FB OPTO GND VCC R18 274Ω 1 2 2 1 Figure 11. Fully isolated, high output voltage regulator High Input Supply Voltage, Non-Isolated Capacitor Charger/Regulator The FB pin of the LT3751 can also be configured for charging a capacitor from a high input supply voltage. Simply tie a resistor divider from the output node to the FB pin. The resistor dividers on the RVTRANS and RDCM pins can tolerate 5% resistors, and all the RV(OUT) and RBG pin resistors are removed. This lowers the number and the tolerance of required components, reducing board real estate and overall design costs. With the output voltage resistor divider, the circuit in Figure 9 is also a fully functional, high-efficiency voltage regulator with load and line regulation better than 1%. Efficiency and line regulation for the circuit in Figure 9 are shown in Figure 10a and Figure 10b, respectively. Alternatively, a resistor can be tied from VOUT to the OVLO1 pin or OVLO2 pin. This mimics the VOUT comparator, stopping charging once the target voltage is reached. The FB pin is tied to ground. The CHARGE pin must be toggled to initiate another charge sequence, thus the LT3751 operates as a capacitor charger only. Resistor R12 is omitted from Figure 9 and resistor R11 is tied from VOUT directly to OVLO1 or OVLO2. R11 is calculated using the following equation: VDRAIN 100V/DIV VDRAIN 100V/DIV GND IPRIMARY 2A/DIV 0 GND IPRIMARY 2A/DIV 0 R11 = VOUT(TRIP) − 1.225 50µA Note that OVLO1 or OVLO2 will cause the FAULT pin to indicate a fault when the target outpaut voltage, VOUT(TRIP) , is reached. High Voltage Input/Output Regulator with Isolation Using a resistor divider from the output node to the FB pin allows regulation but does not provide galvanic isolation. Two auxiliary windings are added to the transformer in circuit shown in Figure 11 to drive the FB pin, the continued on page 42 20µs/DIV 20µs/DIV a. IOUT = 225mA b. IOUT = 7.1mA Figure 12. Switching waveforms Linear Technology Magazine • March 2009 13 L NEW DEVICE CAMEOS battery whether external or internal. Programming the charge current only requires a single external resistor. The fault management system of the LTC4012 family suspends charging immediately for various conditions. First is battery overvoltage protection, which can occur with the sudden loss of battery load during bulk charge. Second, each IC features internal over-temperature protection to prevent silicon damage during elevated thermal operation. The LTC4012 family has a logic-level shutdown control input and three open-drain status outputs. First is an input current limit (ICL) status flag to tell the system when VIN is running at over 95% of its current capacity. The input current limit accuracy is typically ±3% and a maximum of ±4% over the full operating temperature range. Next is the AC present status, which indicates when VIN is within a valid range for charging under all modes of operation. The last is a charge status output can indicate bulk or C/10 charge states. The control input and status outputs of the LTC4012, along with the analog current monitor output, can be used by the host system to perform necessary preconditioning, charge termination and safety timing functions. 4MHz Synchronous StepDown DC/DC Converter Delivers up to 1.25A from a 3mm × 3mm DFN The LTC3565 is a high efficiency synchronous step-down regulator that can deliver up to 1.25A of continuous output current from a 3mm × 3mm DFN (or MSOP-10E) package. Using a constant frequency of (up to 4MHz) and current mode architecture, the LTC3565 operates from an input voltage range of 2.5V to 5.5V making it ideal for single cell Li-Ion, or multicell Alkaline/NiCad/NiMH applications. It can generate output voltages as low as 0.6V, enabling it to power the latest generation of low voltage DSPs and microcontrollers. An independent RUN pin enables simple turn-on and shutdown. Its switching frequency is user programmable from 400kHz to 4MHz, enabling the designer to optimize efficiency while avoiding critical noise-sensitive frequency bands. The combination of its 3mm × 3mm DFN-10 (or MSOP-10) package and high switching frequency keeps external inductors and capacitors small, providing a very compact, thermally efficient footprint. The LTC3565 uses internal switches with an RDS(ON) of only 0.13Ω (N-Channel lower FET) and 0.15Ω (P-Channel upper FET) to deliver efficiencies as high as 95%. It also utilizes low dropout 100% duty cycle operation to allow output voltages equal to VIN, further extending battery run time. The LTC3565 utilizes Automatic Low Ripple ( < 25mVP–P) Burst Mode® operation to offer only 40µA no load quiescent current. If the application is noise sensitive, Burst Mode operation can be disabled using a lower noise pulse-skipping mode, which still offers only 330µA of quiescent current. The LTC3565 can be synchronized to an external clock throughout its entire frequency range. Other features include ±2% output voltage accuracy and over-temperature protection. L LT3751, continued from page 13 Conclusion The ability to run from any input supply voltage ranging from 4.75V to greater than 400V and the abundance of safety features make the LT3751 an excellent choice for high voltage capacitor chargers or high voltage regulated power supplies. In fact, the LT3751 is, for now, the only 42 100 0.5 OUTPUT VOLTAGE ERROR (V) 95 EFFICIENCY (%) LT3751 controller, and the optocoupler on the feedback resistor divider. The auxiliary windings provide the desired galvanic isolation boundary while maintaining an isolated feedback path from the output node to the LT3751 FB pin. Figures 12 and 13 show the regulator’s performance. The fully isolated, high voltage input/output regulator yields over 90% efficiency. Load regulation is excellent as shown in Figure 13b, due mainly to the added gain of the optocoupler circuit. 90 85 80 POUT = 63W POUT = 48W POUT = 25W 75 70 100 120 140 160 180 200 INPUT DC VOLTAGE (V) a. Efficiency 0.25 0 –0.25 –0.5 0 50 100 150 200 250 IOUT (mA) b. Load regulation Figure 13. Fully isolated, high voltage regulator performance boundary-mode capacitor charger controller that can accurately operate from extremely high input voltages. The LT3751 simplifies design by integrating many functions that—due to cost and board real-estate—would otherwise not be realizable. Although several designs are shown here, the LT3751 includes many more features than we can show in one article. We recommended consulting the data sheet or calling the Linear Technology applications engineering department for more in-depth coverage of all available features. L Linear Technology Magazine • March 2009