BADAL ASHUTOSH (IN)
PARASHAR SANKET (IN)
| CN205092791U | 2016-03-16 | |||
| US9490697B2 | 2016-11-08 |
| WE CLAIM 1. A circuit system comprising a passive circuit, a chopper leg and a bipolar gate driver; the passive circuit drive the top switch of the chopper leg from the bipolar gate driver without requiring isolated power supply; wherein the passive circuit comprises bootstrap circuits for positive voltage and negative voltage. 2. The circuit system as claimed in claim 1, wherein the bootstrap circuit for positive voltage comprises a resistor (Ri), capacitor (CBP) and Diode (Di) and bootstrap circuit for negative voltage comprises a resistor (R2), capacitor (CBN) and Diodes (D2, D3, D4, D5) and Zener Diode (ZNI). 3. The circuit system as claimed in claim 1, wherein the bootstrap circuit for negative voltage does not interfere with the positive voltage to drive top switch. 4. The circuit system as claimed in claim 1, wherein the chopper leg is selected from a group of power converters comprising inverter, synchronous buck converter and the like. 5. The circuit system as claimed in claim 1, wherein the bipolar gate driver act as a current booster and voltage translator circuit. 6. The circuit system as claimed in claim 1, wherein the switches ( Ql and Q2 ) of the chopper leg is selected from a group comprising Normally on MOSFETs, insulated gate bipolar transistors, Normally off SiC or eGaN MOSFETS. 7. A method to drive top switch of a chopper leg of circuit system of claim 1, said method involving bootstrap circuit for positive voltage comprises a resistor (Ri), capacitor (CBP) and Diode (Di); wherein top switch bootstrap capacitors CBP of gate driver is charged to positive voltage Vp through diode Di and resistor Ri when bottom MOSFET Q2 is on to apply voltage Vp across gate source of top switch to turn on. 8. A method to drive top switch of a chopper leg of circuit system of claim 1 , said method involving bootstrap circuit for and bootstrap circuit for negative voltage comprises a resistor (R2), capacitor (CBN) and Diodes (D2, D3, D4, D5) and Zener Diode (ZNI); wherein the top switch bootstrap capacitor CBN of gate driver is charged to negative voltage VN through diodes D4, D2 and resistor R2 when top switch Qi is on and bottom switch Q2 is off. 9. The method as claimed in claim 8, wherein the Zener diode ZNI, diodes D3, D5 provides negative voltage to turn the top switch Qi off. |
The present invention is related to the field of electronics, to a power converter. In particular to Gate driver technology of a synchronous buck converter, more specifically to a circuit with passive elements to drive the top switch with a bipolar drive signal without using any isolated power supplies.
BACKGROUND
Wide band gap devices such as SiC based JFETs and GaN based high electron mobility transistors (HEMT) have been identified as promising power semiconductor devices due to their excellent switching characteristic along with very low on state voltage drop. Peftitsis et al; in "Self-powered gate driver for normally on silicon carbide junction field -effect transistors without external power supply" IEEE transaction on power electronics 28.392013): 1488-1501 discusses about normally on SiC JFETs that are commercially available at relatively high voltage rating. Ishibashi, Takaharu et al, in document "Experimental validation of normally -on GaN HEMT and its gate driver circuit." IEEE transactions on Industry Applications 51.3(2015):241502422 has introduced a normally on GaN MOSFET, however this requires a negative voltage to turn the devices off.
Unlike normally off MOSFETs, normally On MOSFETs require gate driver with bipolar voltage capability. The switch is on at zero gate source voltage. A negative voltage is required to turn it off. A positive gate source voltage is required to push it deep into saturation so that the conduction loss is minimal. Normally off Silicon based IGBT and Silicon carbide based MOSFETs also require bipolar gate drive signal. A chopper leg consists of two series connected power semiconductor devices. A chopper leg or a synchronous buck converter is widely used in point of load converters (PoL), DC to DC converters with bi-directional power flow, DC to AC inverters and AC to DC active rectifiers. Isolated power supplies are required to drive the top switch of the chopper leg with bipolar gate drive.
A method to generate bipolar or unipolar negative gate voltage waveform using a unipolar positive gate driver supplied by a non-isolated positive power supply to drive the bottom switch of a chopper leg is presented in "A gate driver for synchronous buck converter with normally on semiconductor switches" Power electronics, Drives and energy systems (PEDES), 2016 IEEE International Conference. The document informs about a top switch being supplied using the same simple bootstrap circuit for synchronous buck converter with normally off MOSFETS, hence no negative or isolated power supply is required in this solution. However, this approach requires more power when compared with a solution that uses bipolar gate drivers, also the said approach involves passive elements and have higher component count resulting in reduced power density and reliability.
US patent No.7248093 proposes a circuit involving active switches to drive the top switch. An isolated power supply is required to drive the top switch but this solution increases the cost and reduces power density and reliability.
Hence, there is a need to develop a circuit and method that is reliable, involve cost effective fabrications, to drive the top switch requiring a bipolar drive signal, without using any isolated power supplies. STATEMENT OF INVENTION:
Accordingly the present invention proposes a circuit with passive elements to drive the top switch requiring a bipolar drive signal, without using any isolated power supplies.
A circuit system comprising a passive circuit, chopper leg and a bipolar gate driver; the passive circuit drive the top switch of the chopper leg from the bipolar gate driver without requiring isolated power supply; wherein the passive circuit comprises bootstrap circuits for positive voltage and negative voltage; a method to drive top switch of a chopper leg of circuit system of present invention, said method involving bootstrap circuit for positive voltage comprises a resistor (Ri), capacitor (C BP ) and Diode (DO; wherein top switch bootstrap capacitors C BP of gate driver is charged to positive voltage Vp through diode Di and resistor Ri when bottom MOSFET Q 2 is on to apply voltage Vp across gate source of top switch to turn on; and a method to drive top switch of a chopper leg of circuit system of present invention, said method involving bootstrap circuit for and bootstrap circuit for negative voltage comprises a resistor (R 2 ), capacitor (C BN ) and Diodes (D 2 , D 3 , D 4 , D5) and Zener Diode (Z NI ); wherein the top switch bootstrap capacitor C BN of gate driver is charged to negative voltage V N through diodes D 4 , D 2 and resistor R 2 when top switch Qi is on and bottom switch Q 2 is off.
BRIEF DESCRIPTION OF FIGURES:
The features of the present invention can be understood with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1 : shows proposed gate driver circuit with power circuit of synchronous buck converter. Figure 2: shows RCD circuit before PWM signal to add dead time in the gate pulse of synchronous buck converter.
Figure 3 : shows top view of the synchronous buck converter with gate driver circuit.
Figure 4: shows Input voltage (5V/div), output voltage (5V/div), pole voltage (lOV/div), output voltage (5V/div), pole voltage (lOV/div), inductor current (ΙΑ/div) of synchronous buck converter.
Figure 5: shows Gate source voltage with turn on transition of top switch (5V/div), and turn off transition of bottom switch (5V/div).
Figure 6: shows Gate source voltage with turn off transition of top switch (5V/div), and turn on transition of bottom switch (5V/div).
DESCRIPTION OF INVENTION:
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular "a" "an" and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention is in relation to a circuit system comprising a passive circuit, chopper leg and a bipolar gate driver; the passive circuit drive the top switch of the chopper leg from the bipolar gate driver without requiring isolated power supply; wherein the passive circuit comprises bootstrap circuits for positive voltage and negative voltage.
In an embodiment of present invention, the bootstrap circuit for positive voltage comprises a resistor (Ri), capacitor (C BP ) and Diode (Di) and bootstrap circuit for negative voltage comprises a resistor (R 2 ), capacitor (C BN ) and Diodes (D 2 , D 3 , D 4 , D5) and Zener Diode (Z NI ).
In another embodiment of present invention, the bootstrap circuit for negative voltage does not interfere with the positive voltage to drive top switch.
In another embodiment of present invention, the chopper leg is selected from a group of power converters comprising inverter, synchronous buck converter and the like.
In another embodiment of present invention, the bipolar gate driver act as a current booster and voltage translator circuit.
In another embodiment of present invention, the switches ( Ql and Q2 ) of the chopper leg is selected from a group comprising Normally on MOSFETs, insulated gate bipolar transistors, Normally off SiC or eGaN MOSFETS.
The present invention is also in relation to a method to drive top switch of a chopper leg of circuit system of present invention , said method involving bootstrap circuit for positive voltage comprises a resistor (Ri), capacitor (C BP ) and Diode (DO; wherein top switch bootstrap capacitors C BP of gate driver is charged to positive voltage Vp through diode Di and resistor Ri when bottom MOSFET Q 2 is on to apply voltage Vp across gate source of top switch to turn on.
The present invention is also in relation to a method to drive top switch of a chopper leg of circuit system of invention, said method involving bootstrap circuit for and bootstrap circuit for negative voltage comprises a resistor (R 2 ), capacitor (C BN ) and Diodes (D 2 , D 3 , D 4 , D5) and Zener Diode (Z NI ); wherein the top switch bootstrap capacitor C BN of gate driver is charged to negative voltage V N through diodes D 4 , D 2 and resistor R 2 when top switch Qi is on and bottom switch Q 2 is off.
In still another embodiment of present invention, the Zener diode Z NI , diodes D 3 , D5 provides negative voltage to turn the top switch Qi off.
This invention presents a method to drive top switch of a synchronous buck converter or a chopper leg implemented with power semiconductor switches that need bipolar gate drive, without any isolated power supply. In general, the invention provides unique circuit with few passive components that can drive the top switch without using any additional isolated power supply or active elements.
The Figure 1 shows the proposed circuit for the gate driver along with the power circuit of a synchronous buck converter with normally on type of MOSFETs Qi and Q 2 . V DC is the input voltage and Vo is the regulated output voltage of the synchronous buck converter. GD is the block diagram of standard bipolar gate driver. GD accepts the control signals for top and bottom switches PWMi and PWM 2 with respect to the system ground. The PWMs are digital signals with voltage levels Vcc and ground and complementary with respect to each other. A RCD circuit, shown in Figure 2, is used to provide a delay in the rising-edge of the PWM signals so that a dead time is generated between the two PWM signals. When the voltage level of the PWM is more than threshold voltage of the gate driver, gate driver will accept it as signal high and vice-versa. The gate driver is essentially a voltage level shifter and a current booster circuit. A pulse with amplitude of Vcc with respect to ground, applied at the input PWM 2 of GD is translated into (V CC2 -V EE2 ) with respect to pin V EE2 at the output pin OP 2 . Output voltage OP 2 with respect to ground will be a bipolar pulse with positive voltage Vp and negative voltage -V N (Vcc 2 is connected to Vp and V EE2 is connected to -V N ). Similarly, for the top switch, a pulse at the input PWMi with respect to ground is translated to a pulse of amplitude (V CCI -V EEI ) with respect to V EEI at the output pin OPi of GD. Bootstrap capacitors C BP and C BN are used to provide the supply to the top driver. The capacitor C BP is charged to Vp through diode Dl and resistance Ri when bottom MOSFET Q 2 is on (pole voltage is zero in this state) and will be applied across gate-source of the top switch to turn it on. The capacitor C BN is charged to -V N through diodes D 4 , D 2 and resistance R 2 when top MOSFET Qi is on (pole voltage is V DC and the diode D 3 reverse biased) and will be applied across the gate-source of top switch to turn it off.
Proposed circuit operates in the following modes:
Mode I. Top switch ON and bottom switch OFF
PWMi is Vcc and PWM 2 is GND which connects OPi to V C ci and OP 2 to V EE2 - Voltage across gate source of Q 2 is -V N - Pole voltage is V DC - D I is reverse biased. CBP will charge the input capacitance Cissl of Qi to the voltage Vp through gate resistance RG1. CBN will be charged to voltage -V N with polarity as shown in figure 1 through diodes D 4 , D 2 and R 2 . D 3 is reverse biased.
Mode II. Top switch OFF and bottom switch OFF (dead time)
For turning off the top switch, PWMi signal is changed to ground which will disconnect OPi from Vcci and connect it to V EEI - The input capacitor Cissl of Qi is initially charged to Vp. In the previous state the pole voltage is same as the DC bus voltage. Due to the output capacitances of the MOSFETs the pole voltage will remain close to the DC bus voltage till the bottom switch is turned on. Voltage at the anode terminal of diode D 2 is sum of pole voltage Vp 0 i e and Vp which will ensure diode D 2 in forward biased. So Cissl will start discharging to asymptotically reach -(Vp 0 ie+VN) until zener Z I breaks down and clamp the voltage across Cissl to negative voltage ( -V N )- This will ensure complete turn off of the top MOSFET Ql.
Mode III. Top switch OFF and bottom switch ON
For turning ON the bottom switch, PWM 2 signal is changed to Vcc which will disconnect OP 2 from V EE2 and connect to Vcc 2 - This will charge Ciss2 from -V N to Vp through gate resistance R G2 . AS bottom switch is ON, pole voltage is zero and CBN is applied across Cissl through RGi and D 3 which discharges Cissl to negative voltage (-V N )- Positive bootstrap capacitor, C BP will start charging through Ri and Q 2 to voltage Vp. Diode D 4 is reversed bias in this mode.
Mode IV. Top switch OFF and bottom switch OFF (dead time)
For turning OFF the bottom switch, PWM 2 signal changed to zero from Vcc which will disconnect OP 2 from Vcc 2 and connect to V EE2 - Ciss2 will be charged to -VN through RG 2 . Till top switch is not ON, body diode of Q 2 will conduct and Vp 0 i e will remain zero.
To exemplify the invention, a synchronous buck converter with commercially available normally on MOSFET, IXTA6N50D2, driven by the proposed gate driver circuit, has been designed, fabricated and tested. A photograph of this converter is shown in the figure 3. Test results at 250 kHz switching frequency, 15V input voltage, 3V output voltage and 2 A output load current (6W output power) are provided in Figure 4. The pole voltage, inductor current and output voltage waveforms confirm the synchronous buck mode of operation of the converter. Figure 5 -6 shows the gate voltage waveforms of both the top and the bottom devices, during the two transitions namely the power to free wheel and free wheel to power. Accordingly it is noted, that the gate waveforms of the top switch are bipolar in accordance with expectation and confirms the operation of the proposed circuit.
The aforesaid description is enabled to capture the nature of the invention. It is to be noted however that the aforesaid description and the appended figures illustrate only a typical embodiment of the invention and therefore not to be considered limiting of its scope for the invention may admit other equally effective embodiments.
Thus the present invention is in relation to a power converter, more specifically to a synchronous buck converter comprising gate driver system with passive elements to drive the top switch without using any isolated power supplies; and provides a simple cost effective solution to the associated problems.