Supertex inc. AN-H59. Application Note

Transcription

1 Supertex inc. AN-H Application Note High Voltage DC/DC Converter for Supertex Ultrasound Transmitter Demoboards By Afshaneh Brown, Applications Engineer, and Jimes Lei, Applications Manager Introduction The Supertex AN-HDB demoboard is a high voltage DC/DC converter. It can provide up to +0V voltage supply for and -0V for V. It also provides +.0 to +0V NN voltage supply for, floating power supplies of +.0 to +0V for V NF and -.0 to -0V for V PF. The input supply voltage is V. The AN-HDB circuitry consists of two high voltage PWM Current-Mode controllers, a DC/DC transformer driver, and three low dropout regulators. The Supertex AN-HDB uses a high-voltage, current mode, PWM controller boost topology to generate + to +0V and a high-voltage current mode PWM controller buck-boost topology to generate - to -0V power supply voltage for Supertex HVDB and HVDB ultrasound transmitter demoboards. Each of the transmitter demoboards has slightly different operating voltages as summarized below. Board V PF V NF HVDB +V -V +.0V -.0V +.0V HVDB +V -V +.0V -.0V +.0V To accommodate all three demoboards, the AN-HDB demoboard has adjustable,,, V PF and V NF. The purpose of the AN-HDB is to aid in the evaluation of the three transmitter demoboards. The intention of this application note is to provide a general circuit description on how each of the output voltages is generated. The VSUB pin on the HVDB and HVDB can either be connected to the most positive supply voltage on the demoboard, or can be left floating. To power up the AN-HDB, ensure that the.v power supply will be powered up first, and then the V power supply. The sequences on the HVDB and HVDB took into consideration using the protection diodes on each power line. The circuit is shown in Figure, the component placement in Figure, and the bill of materials is at the end of this application note. Application Circuit Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

2 AN-H Circuit Description The circuit in Figure shows U, the Supertex high voltage current mode PWM controller, being used to generate the high voltage power supply for. The maximum output power for was set for 0mA at 0V, which is 00mW. With an input voltage of V, a converter frequency of 0 khz with a 00µH inductor was chosen to provide the desired output power. The converter frequency is set by an external resistor, R0, across OSC IN and OSC pins of U. A kω resistor will set the frequency to about 0 khz. R is the current sense resistor..ω was used to set the maximum peak current limit to about 0mA. An RC filter, R and C, is added between the current sense resistor and the current sense terminal pin of U. This reduces the leading edge spike on R from entering the current sense pin. Inductor L is being charged from the V input by M. When M turns off, the energy in L is discharged into C, which is the output through D. The voltage is divided down by feedback resistors R, R, and R. The wiper of R is connected to pin of U. The overall converter will regulate the voltage on pin to.0v. Different output voltages can be obtained by adjusting R. When the wiper for R is set to the top, can be calculated as: = V FB x ( R + R + R ) R + R where V FB is.0v =.0V x ( k + 00k +.k ) =.V 00k +.k When the wiper for R is set to the bottom, can be calculated as: = V FB x ( R + R + R ) R =.0V x ( k + 00k +.k ) =.V.k By adjusting potentiometer R, meets the adjustable target range of to 0V. Comparator U will turn on D when the output is out of regulation due to excessive load. During initial power up, C will be at 0V. D is therefore expected to be on until C is charged to the desired regulation voltage. Figure : Adjustable Power Supply = V R0 kω C OSCIN OSC L 00µH D MMBD D R.kΩ U LM0 + R kω C 0.µF C.0µF 0 FB U HV0NG BIAS VREF DISCH COMP SENSE RESET R.0kΩ C 0pF M TN0 R.Ω R kω R 00kΩ R.kΩ C.µF +V to +0V Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

3 AN-H Circuit Description The circuit in Figure shows U, the Supertex high voltage current mode PWM controller, being used to generate the high voltage power supply for. The function of U is very similar to what was described in the circuit description for U. However, in this circuit a negative voltage is generated from a positive input voltage source, therefore requiring a buck-boost topology. The maximum output power for was set for -0mA at -0V which is 00mW. With an input voltage of V, a converter frequency of 0 khz with a 00µH inductor was chosen to provide the desired output power. Inductor L is being charged from the V input by the parallel combination of M and M. When M and M turn off, the energy in L is discharged into C, which is the output through D0. M and M are high voltage P-channel MOSFETs. U is designed to drive a high voltage N-channel MOSFET. The drive output for U therefore needs to be inverted. This is accomplished by M and M. The feedback voltage that U detects on pin is +.0V. The that needs to be sensed is a negative voltage. A circuit is needed to make sure the feedback voltage is positive. This is consists of Q, Q, R, R, R, R, and R. Q becomes a constant current sink set by the voltage and R. The same current will be flowing through R and R. The voltage on the base of Q will be minus the voltage drop across the sum of R and R. By varying R, the base voltage on Q will change. Q becomes a constant current source with a value set by its base voltage and R. The current source of Q is going into R, which creates a positive voltage that is now proportional to the magnitude of. = V BE - ( R ) x (V BE + V FB x R ), R + R R where V BE = 0.V, V FB =.0V. When R is set to 00k, is calculated to be: = 0.V - ( k ) x (0.V +.0V x.k ).k + 00k 0.k = -.0V When R is set to 0k, is calculated to be: = 0.V - ( k ) x (0.V +.0V x.k ).k + 0k 0.k = -.V By adjusting potentiometer R, meets the adjustable target range of - to -0V. Comparator U will turn on D when the output is out of regulation due to excessive load. During initial power up, C will be at 0V. D is therefore expected to be on until C is charged to the desired regulation voltage. Figure : Adjustable Power Supply = V R kω C0 C R.kΩ R.kΩ D R0.kΩ = V U LM0 C + C.0µF R kω C 0.µF 0 OSC IN OSC U HV0NG BIAS VREF DISCH SENSE COMP FB RESET M TP0K M TN0K R.0kΩ C 0pF M, M TP0N X D0 MMBD L 00µH R.Ω R 00kΩ R kω C.µF R 0kΩ Q FMMT R 0.kΩ Q FMMT VNN -V to -0V Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

4 AN-H VPF and VNF Circuit Description The three transmitter demoboards require two floating low voltage supplies, V PF and V NF. The floating supplies need to be adjustable to accommodate the different operating requirements for the three different boards. The V PF is.0 to 0V below the high voltage positive supply. The V NF is.0 to 0V above the high voltage negative supply. The two floating supplies are generated by using two isolated transformers, T and T, and an isolated transformer driver, U, as shown in Figure. Both outputs utilize adjustable low dropout linear regulators, U and U, as shown in Figure. U and U are both Linear Technology LT, which has a reference voltage of.v on the pin. For V PF, resistors R, R, and R set the output V PF voltage. R is a potentiometer for adjusting V PF. V PF can be calculated with the following equation: V PF = V x ( R + R + R ) R + R When R is set to 0kΩ, V PF becomes: V PF =.V x (.k + 0k +.k ) =.V 0k +.k When R is set to 0Ω, V PF becomes: V PF =.V x (.k + 0k +.k ) = 0.V 0 +.k Please note that the pin on U is referenced to, thereby setting V PF to be.0 to 0V below. V NF can also be calculated in a similar manner using resistors R, R, and R. Please note that the pin on U is referenced to thereby setting V NF to be.0 to 0V above. indicators, D and D, start to turn on when the input current to U and U reaches an arbitrary value of 0mA. This is set by Q and R for V PF and Q and R for V NF. The input current can be calculated with the following equation: Input current = V EB = 0.V =.ma R.Ω 0mA current limits are added to protect against output shorts. The current limiter is consists of a depletion-mode MOSFET and a series source resistor. The resistor sets the current limit and can be estimated with the following equation: R SERIES = V TH x ( I LIM / I DSS - ) where, I LIM V TH = pinch-off voltage for M and M: -.V I LIM = desired current limit: 0mA I DSS = saturation current for M and M:.A R SERIES =.Ω. A 0.Ω resistor was used. Figure : Adjustable V PF and V NF Power Supply = V C R.kΩ R.kΩ C 0µF 0, COLA COLB RSL U LT RT CT SYNC P D CTX0-0 D CTX0-0 D MMBD D MMBD D MMBD D MMBD R.Ω Q FMMT C R.Ω C R.kΩ Q FMMT R.kΩ M DN R.kΩ D M DN R0.kΩ D R 0.Ω R 0.Ω R 00kΩ C R0 00kΩ C,, IN U LT IN U LT,, R.kΩ R 0kΩ R.kΩ R.kΩ R 0kΩ R.kΩ +.0 to +0V C +.0 to +0V C VPP VFP VNF VNN Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

5 AN-H Circuit Description The output voltage utilizes an adjustable low dropout linear regulator, U LT, as shown in Figure. The desired adjustable output voltage range is.0 to 0V to accommodate the different operating voltages for the three different transmitter demoboards. The LT has a reference voltage of.v on the adj pin. Resistors R, R, and R set the output voltage. R is a potentiometer for adjusting. can be calculated with the following equation: = V x ( R + R + R ) R + R When R is set to 0kΩ, becomes: =.V x (.k + 0k +.k ) =.V 0k +.k When R is set to 0Ω, becomes: =.V x (.k + 0k +.k ) = 0.V 0 +.k An indicator, D, is included in case of excessive input, I IN, current. D is starts to turn on when the input current reaches an arbitrary value of 0mA. This is set by Q and R. When the emitter-base junction of Q is forward biased (0.V), Q will start to turn on, thereby forward biasing D. The I IN value to turn D on can be calculated with the following equation: I IN = V EB = 0.V = 0.mA R.Ω Figure : Adjustable Power Supply = V R.kΩ C Q FMMT R.kΩ D R 00kΩ C,, IN U LT R.kΩ R 0kΩ R.kΩ C0 +.0 to 0V.V Input Terminal The AN-HDB has a.v input terminal that is directly connected to the output terminal, V CC. There is no circuitry on this board that uses the.v supply. It is only there as a convenient connection to the -pin header. V CC is the logic supply voltage for HVDB and HVDB and can operate from. to V. However, most users will operate V CC at either.0 or.v. Input and Output Power The output voltages from the AH-HDB are all generated from the V input line. With no load on the outputs, the measured input current was about 0mA. This input current can vary from board to board due to variations in the isolated transformer. The maximum output power is: P (MAX) = P VPP(MAX) + P VNN(MAX) + P VPF(MAX) + P VNF(MAX) + P (MAX) P (MAX) = 0.W + 0.W + 0.W + 0.W + 0.W P (MAX) =.W Under this condition, the V input current was measured to be 0mA. Input power is therefore.0w. This gives an approximate overall efficiency of % at full load. Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

6 AN-H VPF and VNF Output Current The AN-HDB can supply more than 0mA of current for the V PF and V NF outputs. The I NF and I PF input currents for the HV or the HV can be found in their respective data sheet but are summarized below: Part # IPF-mode INF-mode HV 0mA ma HV 0mA ma This is for continuous.0 MHz operation. For ultrasound, the high voltage transmitter is operating at very low duty cycles; % or lower. At a % duty cycle, the average current is expected to be a 00 times lower. The 0mA output current capability on the AN-HDB is more than sufficient to power up the HV or the HV. Conclusion The main purpose of AN-HDB power supply demoboard is to help the evaluation of the Supertex HVDB and HVDB demoboards by reducing the number of power supplies needed. The AN-HDB was designed to operate from a single V input which should be commonly available in any engineering laboratory. The five on-board s allow the user to quickly determine whether there is an overload condition on each of the supply lines. The five potentiometers allow the user to easily adjust each supply to meet their particular needs. Figure : AN-H Component Placement Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

7 Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: AN-H Figure : AN-H Circuit Schematic 0 SENSE OSC IN OSC RESET U HV0NG FB BIAS N/C COMP VREF COLA COLB RSL RT CT SYNC P IN,,,,,, R kω R 00kΩ R.kΩ R.kΩ R 0kΩ R.kΩ R.kΩ R 0kΩ R.kΩ R 0kΩ R.kΩ R.kΩ R.Ω R.kΩ R.kΩ R.kΩ R.kΩ R.kΩ R0.kΩ R.kΩ R.kΩ R 00kΩ IN R0.kΩ R 0.kΩ R0 00kΩ R 00kΩ R.kΩ R0 kω R kω R.0kΩ R 0.Ω R.Ω R.kΩ R.kΩ R kω R kω R.0kΩ R.Ω R 0kΩ C C0 C.µF C.0nF C C C C C0 C 0 SENSE OSC IN OSC RESET U HV0NG FB BIAS N/C COMP VREF R 00kΩ C C C R 0.Ω R.Ω C C C 0pF C.µF C 0pF C 0.µF C C 0pF C 0.µF C 0µF Q FMMT Q FMMT Q FMMT Q FMMT Q FMMT L 00µH D D0 MMBD R kω D D MMBD D MMBD D MMBD D MMBD D D MMBD D M TN0 M, M TN0 M TN0 M TN0 M DN M DN U LM0, 0 VPP VPF VNF VNN VCC + - U LM0 + - T CTX0-0 T CTX0-0 U LT U LT U LT IN U LT VPP VPF VNF VNN VCC J L 00µH V.V D

8 AN-H Bill of Materials Reference Description Package Manufacturer Part No. C,,,,,,,,, 0,,, 0, Chip Capacitor,, V 0 Any --- C,, Chip Capacitor, 0pF, 00V 0 Any --- C, Chip Capacitor, 0.µF, V 0 Any --- C, Chip Capacitor,.0nF, 0V 0 Any --- C, Chip Capacitor,.µF, 00V 0 Any --- R,.kΩ, Chip Resistor 0 Any --- R,.Ω, Chip Resistor 0 Any --- R, 0.Ω, Chip Resistor 0 Any --- R, 0,.kΩ, Chip Resistor 0 Any --- R,,.kΩ, Chip Resistor 0 Any --- R,, 0kΩ, Potentiometer --- Any --- R,,.kΩ, Chip Resistor 0 Any --- R.Ω, Chip Resistor 0 Any --- R,, 0.kΩ, Chip Resistor 0 Any --- R0, kω, Chip Resistor 0 Any --- R, kω, Chip Resistor 0 Any --- R,.0kΩ, Chip Resistor 0 Any --- R,.0Ω, Chip Resistor 0 Any --- R kω, Chip Resistor 00 Any --- R, 00kΩ, Potentiometer --- Any --- R.kΩ, Chip Resistor 00 Any --- R kω, Chip Resistor 00 Any --- R 0.kΩ, Chip Resistor 00 Any --- R.kΩ, Chip Resistor 00 Any --- R 0.kΩ, Chip Resistor 00 Any --- R, 0, 00kΩ, Chip Resistor 00 Any --- R,.kΩ, Chip Resistor 0 Any --- L, Inductor, 00µH --- Cooper Electronic SD-0-R D,,,,, 0 00V, Fast Recovery Diode SOT- Fairchild MMBD D,,,, Red 00 Lumex SML-LXT00SRW Q,,, PNP, 0V, Bipolar Transistor SOT- Zetex Inc FMMTTA Q NPN, 0V, Bipolar Transistor SOT- Zetex Inc FMMTTA U IC, Low Noise Transformer Driver -TSSOP Linear Technology LTEFE#PBF U,, IC, Adjustable Linear Regulator SO- Linear Technology LTCS#PBF Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--

9 AN-H Bill of Materials (cont.) Reference Description Package Manufacturer Part No. U, High-voltage current-mode PWM controller SO- Supertex Inc. HV0NG-G U, IC, Dual Voltage comparator SO- Texas Instruments LM0DR T, Transformer --- Cooper Electronic CTX0-0 M, M M M M, J MOSFETs Depletion Mode, N-channel, 0V MOSFET Enhancement Mode, N-channel 00V MOSFET Enhancement Mode, P-channel 0V MOSFET Enhancement Mode, N-channel 0V MOSFETs Enhancement Mode, P-Channel 00V Position, 0.00 Pitch, rectangular connector SOT- Supertex Inc. DNN-G SOT- Supertex Inc. TN0N SOT- Supertex Inc. TP0K SOT- Supertex Inc. TN0K SOT- Supertex Inc. TP0N --- Tyco Electronic Amp 00- 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// 0 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. 0 Supertex inc. Bordeaux Drive, Sunnyvale, CA 0 Tel: 0--