Application Note AN-1087 Design of Secondary Side Rectification using IR1167 SmartRectifier™ Control IC By Maurizio Salato, Adnaan Lokhandwala, Marco Soldano Table of Contents Device Overview SmartRectifier™ Concept Operation and analysis in CCM & DCM Flyback Operation in Resonant Converters Typical System Schematics and Passive Components Nomenclature Design Procedure & example PCB Layout Guidelines Symbols list www.irf.com AN-1087 1 Introduction IR1167S is a smart secondary-side driver IC designed to drive N-Channel power MOSFETs used as synchronous rectifiers in isolated Flyback converters. The IC can control one or more paralleled MOSFETs to emulate the behavior of Schottky diode rectifiers. The drain to source voltage of the MOSFET is sensed differentially to determine the level of the current and the device is turned on and off in close proximity of the zero current transition. The pinout for the 8 pin device is shown below. Figure 1: IR1167 SmartRectifier™ control IC pin assignment. SmartRectifier™ Concept The typical power stage schematic for a flyback converter with synchronous rectification (low-side configuration) at the output is shown below. Vin XFM Cs Co LOAD Rs Ci M2 M1 Rtn Figure 2: Typical schematic of a flyback converter with synchronous rectification. www.irf.com AN-1087 2 When the primary device M1 is turned off, the secondary current will flow through the body diode of the MOSFET M2 (which is analogous to the current flowing through the output diode when diode rectification is used). The SmartRectifier™ Control Technique is based on sensing the voltage across the MOSFET and comparing it with two negative thresholds to determine the turn on and off transition for the device. A higher negative threshold, VTH2, detects current through the body diode and hence, controls the turn on transition for the power device. Similarly, a second externally programmable smaller negative threshold, VTH1, determines the level of the current at which the device turns off. VGate VDS VTH2 VTH1 VTH3 Figure 3: IR1167 SmartRectifier™ control IC differential voltage sensing thresholds. Additional control logic has been incorporated to prevent false turn off and gate chattering when the device current transitions between its body diode and channel. When the power device is turned on, the instantaneous sensed voltage reduces to RDSon ⋅ I D and depending on the level of the device current, could fall below the turn off threshold and cause false device turn off. Additionally, the device turn on is also associated with some parasitic ringing between the transformer leakage inductance and device output capacitance. Operation and analysis in CCM & DCM Flyback The IR1167 SmartRectifier™ IC can emulate the operation of diode rectifier by properly driving a Synchronous Rectifier (SR) MOSFET. The rectifier current is sensed by the input comparator using the power MOSFET RDSon as a shunt resistance and the GATE pin of the MOSFET is driven accordingly depending on the level of the sensed voltage with respect to the 3 thresholds shown in Figure 3. Internal blanking logic is used to prevent spurious transitions and guarantee operation in continuous (CCM), discontinuous (DCM) and critical (CrCM) conduction mode. www.irf.com AN-1087 3 The modes of operation for a Flyback circuit differ mainly for the turn-off phase of the SR switch, while the turn-on phase of the secondary switch (which correspond to the turn off of the primary side switch) is identical. ¾ Turn On Phase When the conduction phase of the SR FET is initiated, current will start flowing through its body diode, generating a negative VDS voltage across it. The body diode has generally a much higher voltage drop than the one caused by the MOSFET on resistance and therefore will trigger the turn-on threshold VTH2. At that point, the IR1167 will drive the gate of MOSFET on which will in turn cause the conduction voltage VDS to drop down. This drop is usually accompanied by some amount of ringing, that can trigger the input comparator to turn off; hence, an externally programmable Minimum On Time (MOT) blanking period is used that will maintain the power MOSFET on for a minimum amount of time. The programmed MOT will limit also the minimum duty cycle of the SR MOSFET and, as a consequence, the max duty cycle of the primary side switch. Figure 4: MOT and tBLANK during operation in DCM. Notice both Minimum On Time and Blanking time logic are allowed only once per switching cycle; it is necessary that VDS reaches VTH3 (therefore primary turn on) for them being enabled again (therefore ready for the next switching cycle). www.irf.com AN-1087 4 ¾ DCM/CrCM Turn Off Phase Once the SR MOSFET has been turned on, it will remain on until the rectified current will decay to the level where VDS will cross the turn-off threshold VTH1. This will happen differently depending on the mode of operation. dI . Once the dt threshold is crossed, the current will once again flow through the body diode, causing the VDS voltage to jump negative. Depending on the amount of residual current, VDS may again trigger the turn on threshold: for this reason VTH2 is blanked for an internally set blank time tblank (as shown in Figure 4) after VTH1 has triggered. As soon as VDS crosses the positive threshold VTH3, this blanking time is terminated and the IC is ready for next conduction cycle. In DCM the current will cross the threshold with a relatively low ID_PRIM VDS_PRIM T1 T2 T3 time ID_SEC VDS_SEC time Figure 5: DCM operating flyback converter simplified waveforms. www.irf.com AN-1087 5 ID_PRIM VDS_PRIM T1 time T2 ID_SEC VDS_SEC time Figure 6: CrCM operating flyback converter simplified waveforms. ¾ CCM Turn Off Phase During the SR FET conduction phase the current will decay linearly, and so will VDS on the SR FET. Once the primary switch will start to turn back on, the SR FET current will rapidly decrease crossing VTH1 and turning the gate off. The turn off speed is more critical here to avoid cross conduction on the primary side and reduce switching losses. The blanking period is also applied in this case, but given the very fast nature of this transition, it will be reset as soon as VDS crosses VTH3. ID_PRIM VDS_PRIM T1 time T2 ID_SEC VDS_SEC time Figure 7: CCM operating flyback converter simplified waveforms. www.irf.com AN-1087 6 Operation in Resonant Converters Figure 8 shows the typical secondary-side schematic for a series resonant converter with a capacitive output filter. Implementing synchronous rectification in such applications would require 2 current sense transformers, 2 high speed comparators and finally 2 high current, low propagation delay gate drivers needed to drive the two power devices. Existing monolithic solutions are based on PLL control techniques and rely on synchronizing signals from the primaryside to anticipate the turn off transition for the secondary MOSFETs; hence, they cannot guarantee reliable operation when the converter operates in burst mode during light and no load conditions. The SmartRectifier™ control technique operates completely independent of the primary-side switching technique and the dI low transitions in the resonant converter make the IR1167 an excellent dt candidate in such applications. Figure 9 illustrates this with waveforms. M1 Ls 1 Lp Ls 2 Cout LOAD M2 Figure 8: Series resonant converter secondary-side general schematic. IR1167 operates completely independent of the primary-side switching technique dI and the low transitions make it an excellent candidate for such applications dt as shown in Figure 9 below. www.irf.com AN-1087 7 VTH3 ID_SEC VDS_SEC T1 T2 time VTH1 VTH2 Gate Drive time Blanking MOT tblank Figure 9: Series resonant converter with IR1167 SmartRectifier™ control IC general waveforms. In resonant converter applications, output voltage regulation can be achieved by operating in fixed or variable frequency (50% duty cycle) operating modes. In variable frequency applications, the converter operates at the minimum switching frequency at low line-full load conditions and at the maximum frequency at high line-no load conditions. Hence, the MOT selection for resonant converters can simply be based on the maximum switching frequency of the converter. MOT ensures proper gating signals for the synchronous MOSFETs during light load conditions (i.e. operating at maximum switching frequency), and the situation only improves when the converter operates at heavier loads. Typical System Schematics and Passive Components Nomenclature Passive components needed for IR1167 operations are: • C: supply decoupling capacitor • Rg: synchronous MOSFET gate resistor • RMOT: Minimum On Time setting resistor Component not necessary but recommended: • RCC: series resistor on supply In all the low side configurations, power can be drawn directly from converters’ output whenever its regulated voltage falls in the recommended range (12-20V). In all other cases, recommendation is to provide a dedicated supply through: www.irf.com AN-1087 8 • Auxiliary transformer winding if high side • Transformer main winding tap if low side Figure 10 to Figure 16 show typical systems schematics. R C Rg Vgate Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Figure 10: single ended, low side rectification, supply from winding tap (Voutput < 12V). Rcc C Rg Vgate Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Figure 11: single ended, low side rectification, supply from output voltage (Voutput = 12-20V). www.irf.com AN-1087 9 C R Rg Vcc Vgate OVT GND MOT Vs EN Vd + - IR1167 Rmot OUTPUT Figure 12: Single ended, high side rectification, supply through aux winding (output voltage independent). IR1167 Rg Vd EN Vs MOT GND OVT Vgate Vcc Rmot C R C Rg Vgate Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Figure 13: Center tap, low side rectification (Half or Full bridge resonant), supply from extra winding (Voutput < 6V). www.irf.com AN-1087 10 IR1167 Rg Rmot Vd EN Vs MOT GND OVT Vgate Vcc C R C Rg Vgate Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Figure 14: Center tap, low side rectification (Half or Full bridge resonant), supply from winding tap (6V < Voutput < 10V). IR1167 Rg Vd EN Vs MOT GND OVT Vgate Vcc Rmot C Rcc Rcc C Vgate Rg Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Figure 15: Center tap, low side rectification (Half or Full bridge resonant), supply from output voltage (Voutput = 12-20V). www.irf.com AN-1087 11 C R Rg Vcc Vgate OVT GND MOT Vs EN Vd + - IR1167 Rmot OUTPUT C R Rg Vcc Vgate OVT GND MOT Vs EN Vd Rmot IR1167 Figure 16: Center tap, high side rectification (Half or Full bridge resonant), supply from extra winding (output voltage independent). Required System Parameters Fundamental values to be captured on the system if not known by design are 1. maximum switching frequency f SWmax and minimum operating switching frequency f SWmin 2. secondary minimum conduction time, also called (in SmartRectifier™ terminology) Minimum On Time ( MOT ) 3. mode of operations: Continuous (CCM), Discontinuous (DCM) or Critical (CrCM) conduction mode of operations 4. The maximum temperature of the environment in which the IR1167S IC will operate, TICamb (this is normally the maximum PCB temperature) 5. The available supply voltage Vsupply . It can be the converter output voltage (for low side rectification systems) or a dedicated supply (i.e.: transformer extra winding or tap) www.irf.com AN-1087 12 ¾ Minimum On Time (MOT) Determination In order to properly capture the minimum on time on an existing system, the following procedure is recommended. First, probe the output rectifier voltage (or current) and set the oscilloscope in order to trigger the conduction waveform. Second, identify the condition (AC main line and load) which visually shows reducing the conduction waveform at minimum (normally small or no load, high line). Then, using measuring tools, monitor the conduction pulse width statistically. Most equipments include that function. Due to variable frequency nature of most flyback controllers, width of that pulse can vary significantly. Target is therefore to capture the lowest possible value while slightly varying both, load and main line, around the identified working point (if the condition is at no load, there is no need to apply some load). Figure 17 shows the acquired statistic - Figure 17: Minimum On Time determination on an existing system. Notice that the minimum value (low) showed in the scope statistic could easily mislead. It can be due to several reasons not directly linked to reality (trigger issues, measuring issues, etc.) and it’s normally the minimum absolute recorded. In this case, the recommendation is to properly use the statistical data, assuming the average minus 6 times the standard deviation reasonably represents the minimum possible value (3ppm). In the present example MOT = 2.32 − 6 ⋅ 0.0987 = 1.73µs www.irf.com AN-1087 13 ¾ Maximum Switching Frequency Determination Like in MOT determination, also max frequency can be variously located around different power supply working conditions. A simple sweep of AC line and load can easily show around which working point this condition occurs. Then using statistical tool, max frequency is easily measured around the identified working point (slight variation of load and main AC line). Figure 18: maximum switching frequency determination on an existing system. Also in this case, low and high value shown on scope could easily misled. Having this parameter thermal impact on IR1167 design only and being this averaged in nature (see electrical procedure), we recommend keeping average plus 3 sigma (0.3%) as a good rule for determining the maximum switching frequency. In the present example: f SWmax = 66.14 + 3 ⋅ 2.48 = 73.6kHz ¾ Mode of Operation Determination Mode of operation can be easily checked sweeping both, AC main line and load across their range. www.irf.com AN-1087 14 Detailed Design Procedure Following procedure assumes the synchronous MOSFET has been already identified as well as the above mentioned systems parameters a. OVT setting Table 1 shows the guidelines in properly choosing the right OVT (turn on and off comparator Offset Voltage Trimming) threshold. System mode of operation OVT connected to (typical values) DCM or CrCM Ground, VTH1= -3.5mV Boundary CCM Floating, VTH1= -10.5mV CCM VCC, VTH1= -19mV Table 1: OVT setting guideline as function of system mode of operations The basic idea behind this is the need to ideally approximate a rectifier behavior, having the voltage sense as a sole input to the controller. In DCM or CrCM, it is obvious the use of the comparator threshold closer to zero, in order to maximize the conduction through the MOSFET channel. In CCM conditions, the reverse voltage normally appears quickly across the rectifier, requiring the controller to turn it off with the minimum possible delay. The two lower comparator thresholds enable earlier detection of current fall. Ideally, the optimized system would have one of those two thresholds as closest as possible to the MOSFET VDS at the end of the conduction cycle as shown in Figure 19. www.irf.com AN-1087 15 Figure 19: OVT design for CCM operations. Further requirements for CCM operations are explained at the end of this design procedure. In boundary CCM (CCM during ac main or load boundary conditions only, therefore rarely occurring in normal system operating conditions) the floating OVT threshold generally represents a good compromise. b. IC current consumption calculation First, from selected synchronous MOSFET total gate charge Qg and gate to drain charge Qgd data have to be identified, together with the corresponding gate voltage Vgs. Because of the IR1167 mode of operations, secondary current initially flows through the body diode, therefore the turn on gate characteristic doesn’t include the Miller charge. Figure 20 shows how a regular gate characteristic (black continuous line) changes when the switch is turned on at zero or slightly negative drain to source voltage (red discontinuous line) www.irf.com AN-1087 16 VGS QG Qgd Qg Qg-Qgd Figure 20: MOSFET gate characteristic when driven by SmartRectifier™ control IC. It is evident much less charge is required and the behavior can satisfactorily be modeled as a capacitor: Csync = Qg − Qgd Vgs If more parts are paralleled, the above capacitance have to be multiplied for the number of devices. The maximum IC required current can then be calculated as following: ( I CC = f SWmax CsyncVghigh + I QCC + 7 ⋅10 −9 f SWmax ) where Vg high is the IR1167 gate driver output voltage and fSW_max is the converter maximum switching frequency as previously identified. The first term is entirely due to synchronous MOSFET gate drive while the second term accounts for the IC internal logic consumption in regular operations (the factor 7 ⋅10 −9 accounts the frequency dependent current requirements for the internal logic). Notice this term is independent from VCC voltage. www.irf.com AN-1087 17 c. Gate and supply series resistors design and thermal verification IR1167S based synchronous rectification has the prerogative to turn the switch on and off at VDS levels close to zero. The gate resistor hasn’t therefore impact on transitions and can be designed on a different basis. In order for the gate loop to be optimized, oscillations have to be avoided in regular operations. Assuming the total gate trace loop inductance is known (a very first estimation can be 1nH/mm of physical trace length), the minimum recommended gate resistor will be Rg loop > 2 Lg Ciss Where Ciss is the switch input capacitance (from switch datasheet). Figure 21 shows how this critical resistance value varies with the overall gate loop inductance for some popular International Rectifier MOSFETs. IR1167 driving stage: minimum required gate resistor vs. gate loop inductance for circuit damping using some popular IR HexFET (0.7Ω IR1167 driver impedance and 1.2Ω FET internal gate resistance included) Critical gate resistance for loop damping [ Ω] 8 IRF7853 Ciss = 1.64nF IRF6644 Ciss = 2.21nF IRFB4610 Ciss = 3.55nF IRFB4410 Ciss = 5.15nF IRFB4110 Ciss = 9.62nF 7 6 5 4 3 2 1 0 5 8 11 14 17 20 23 26 29 32 35 Total gate loop inductance [nH] Figure 21: minimum external gate resistor vs. gate loop inductance for some MOSFETs. It is evident how a good layout practice can dramatically reduce this requirement. www.irf.com AN-1087 18 Now, let’s consider the well known series RC network transient: the energy dissipated by the resistor is exactly equal to the energy stored in the capacitor. IR1167S internal gate driver is of course always in series with the external gate resistor, which means they will linearly share the power dissipation. First, let’s calculate the energy stored in the MOSFET gate: Eg = 1 CsyncVg2high 2 The power dissipated by the driver buffer AND the total gate resistance will therefore be Pdr = 2 f SW max E g The driver buffer and the total gate resistance will linearly share this power dissipation as described in the following relationship: Rg Rg + PRg = R +R Rg + RSink Source g Pdr ⋅ 2 Rearranging this last relationship Rg Rg 1 = + Pdr 2 Rg + RSource Rg + RSink PRg and solving it with respect to Rg (which includes the external gate resistor and the MOSFET internal gate resistance), it is possible to plot the percentage of the total driving power dissipated into the gate resistor as a function of its value. This is also useful for proper dimension the gate resistor itself. Notice on IR1167S datasheet, pull up and pull down resistances are defined; while RSink = rdown , RSource = 1.1 rup in order to account for some extra energy dissipated for voltage clamping. www.irf.com AN-1087 19 IR1167 driving stage: percentage of the required driving power Pdr dissipated in the gate resistor as a function of the gate resistor value Percentage of Pdr dissipated in gate resistor [W] 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Gate resistor value [Ω] Figure 22: percentage of gate driver power dissipated in the gate resistor as function of its value. It is evident the asymptotic nature of the curve (it would require an infinite gate resistor for dissipating all the power in it). The final step is the thermal verification for the chosen value. Using the maximum thermal resistance junction to ambient, the maximum temperature (where ambient refers to the environment in which the IC will work , i.e. box, PCB etc.) and the IC maximum junction temperature, it is now possible to calculate the maximum allowable IC power dissipation PICmax = TJ max − TIC _ amb RϑJA where, according to IR1167 datasheet, RθJA=128ºC/W. Because PRg is known and supply current has already been calculated, this will imply to limit the VCC supply voltage (therefore the maximum input power for IR1167) VCCmax = www.irf.com PICmax + PRg I CC AN-1087 20 Figure 23 shows maximum allowable IR1167A VCC vs. maximum switching frequency for some popular International Rectifier 100V MOSFET, assuming 1Ω external gate resistor and 85ºC environment IR1167AS maximum VCC vs. synchronous rectifier switching frequency TIC_J=125ºC, TIC_amb=85ºC external RG=1Ω, HexFET 1Ω gate resistance included 20 Maximum allowable VCC voltage [V] 19 18 17 16 IRF6644 Csynch = 2.4nF 15 IRFB4610 Csynch =5.4nF IRFB4410 Csynch = 7.6nF 14 IRFB4110 Csynch = 10.8nF 13 12 11 50 100 150 200 250 300 350 400 450 500 Maximum synchronous HexFET switching frequency [kHz] Figure 23: Max VCC supply voltage vs. switching frequency for some chosen MOSFET, IR1167AS IC ∆TJ=40ºC. In order to avoid UVLO issues, VCC designs below 12V should be avoided. It is clear how supply voltage and gate resistor play a major role in the design trade off. In most commercial systems, the minimum gate resistor value for loop damping will satisfy the thermal requirements. If not, the procedure has to be iterated taking the following steps Step 1: decrease the VCC to the lowest possible value through a series resistor: RCC = Vsupply − VCC I CC If this allows VCC to comply the thermal limit, then gate resistor value can be kept as designed. www.irf.com AN-1087 21 It is worth mentioning the additional benefit of adding some series resistance to supply: an enhanced filtering effect with local decoupling capacitor. For systems powered from the output (no dedicated power through windings, etc.) this can result in smoother operations Step 2: increase the gate resistor value. This can be of some effect if a small resistance has been used, according to Figure 22. d. Decoupling capacitor Several techniques are possible for decoupling capacitor sizing, depending upon system topology and or special requirements. As general guideline a capacitor of at least 100nF should be used. The two most common cases are IR1167S powered directly form the output or from a dedicated winding. In the first case, in order to reduce the voltage ripple and possible noise, a good criteria is to use a series resistor on supply (if not already used for thermal management reasons) and size the capacitor in order to obtain a low pass filter with pole frequency a couple of octaves below the minimum operating switching frequency (not stand-by) 2 C min = π ⋅ f SWmin ⋅ RCC Figure 24 chart shows some obtained values as examples. A minimum value of 100nF limits the curves. www.irf.com AN-1087 22 Decoupling capacitor value vs minimum switching frequency (4 RCC values) for 10dB attenuation (two octaves on frequency domain) 2.0E-06 1.8E-06 Rcc = 10ohm Rcc = 33ohm Rcc = 66ohm Rcc = 100ohm Decoupling capacitor value [C] 1.6E-06 1.4E-06 1.2E-06 1.0E-06 8.0E-07 6.0E-07 4.0E-07 2.0E-07 0.0E+00 10 60 110 160 210 260 Converter minimum operating frequency [kHz] Figure 24: decoupling capacitor value vs. min. switching frequency, 10dB attenuation on supply. In case of operations through aux winding or winding tap, decoupling capacitor should be sized in order to allow one switching period operation even in absence of main supply, within an acceptable voltage ripple ∆VCC C min = I CC f SWmin ⋅ ∆VCC e. MOT resistor Being the MOT setting linear with the resistor value, the following relationship can be used RMOT = 2.5 ⋅ 1010 t MOT www.irf.com AN-1087 23 f. Rectifier turn off maximum current slope calculation for CCM systems dI SEC dt turnoff when the SmartRectifier™ turns off, in order to maximize efficiency. Figure 25 shows turn off waveforms, where at given times: • t1: primary switch turns on • t2: secondary VDS hits VTH1 threshold • t3: secondary VDS and ID reach zero In CCM systems, it is highly recommended to control secondary slope dI SEC dt turn off ID_SEC time VTH1 VDS_SEC tDoff + 3 gate off SR Gate Drive t1 t2 t3 time Figure 25: SmartRectifier™ turn off waveforms in dI/dt controlled CCM conditions. It is evident, the optimal condition is to have synchronous FET turned off when dVDS the current approaches zero. In order to obtain this, should be designed dt such as enough time is allowed to internal logic to react (tDoff on IR1167 datasheet) and to gate driver to completely discharge the gate (3 times the gate loop time constant at turn off, i.e. τ gate off = ( Rg FET + Rg + rdown ) ⋅ Csync , where resistances are internal MOSFET gate resistance, external gate resistor and driver pull down resistance). This normally ends up to be in the range of 55 – 100ns. The secondary current slope at turn of should therefore be designed according to www.irf.com AN-1087 24 dI SEC VTH 1 ≤ dt turnoff RDS on (t Doff + 3τ gate off ) In order to obtain this, a small saturable core in series with the primary winding could be used or a sufficiently rugged primary transistor with slow turn on for small power systems. The primary maximum current slope requirement at turn on will be easily calculated using transformer turns ratio: dI PRI N dI = SEC ⋅ SEC dt turnon N PRI dt turnoff If this design condition is met, the reverse current through the Synchronous FET will be minimal and only needed for charging its output capacitance up to the reverse voltage. Design example with IR1167AS (10.7V gate output voltage) System data: • f SWmax = 250kHz • MOT = 1.2µs • • Critical Conduction Mode f SWmin = 18kHz • TICamb = 80º C • Low side rectifier system, output voltage 19V (direct supply from converter output possible) Synchronous MOSFET: IRFB4110, 100V 4.5mΩ max • Qg = 150nC @ Vgs = 10V • Qgd = 43nC @ Vgs = 10V • Ciss = 9.62nF ; Rg FET = 1.3Ω www.irf.com AN-1087 25 a. OVT setting: ground b. IC current consumption calculation Csync = Qg − Qgd Vgs = 10.7nF ( ) I CC = f SWmax CsyncVghigh + I QCC + 7 ⋅10−9 f SWmax = 32.8mA c. Gate and supply series resistors design and thermal verification Assuming the total gate loop trace length is 15mm (0.6inch) therefore Lg ≈ 15nH Rg loop > 2 Lg Ciss = 2.5Ω From MOSFET datasheet internal gate resistance is 1.3Ω, from IR1167S datasheet driver pull down resistance is 0.7Ω, for a total of 2Ω. It looks therefore reasonable to chose an external gate resistor for the missing part Rg = 0.5Ω According to the procedure, let’s now verify the system thermally: Pdr = 2 f SW max E g = 306mW Therefore Rg + Rg FET Rg + Rg FET + PRg + PRg _ FET = Rg + Rg + RSource Rg + Rg + RSink FET FET Pdr ⋅ = 155mW 2 Assuming an acceptable IC maximum junction temperature of 130ºC PICmax = www.irf.com TJ max − TIC _ amb RϑJA AN-1087 = 390mW 26 Which means a maximum VCC voltage VCCmax = PICmax + ( PRg + PRg _ FET ) I CC = 16.6V This value is far away from the available 19V supply, and a series resistor would probably dissipate too much power. According to Figure 22, it is possible to slightly increase the gate resistor and get significant benefits. Let assume it is increased in value to Rg = 1.1Ω The power dissipated in the gate loop resistance external to the IC will become Rg + Rg FET Rg + Rg FET + PRg + PRg _ FET = Rg + Rg + RSource Rg + Rg + RSink FET FET Pdr ⋅ = 172mW 2 The maximum supply voltage will change to VCCmax = PICmax + ( PRg + PRg _ FET ) I CC = 17.2V It is reasonable now to “fill the gap” with 19V through a series resistor: RCC = 55Ω This resistor will dissipate a maximum of 60mW, which is considered acceptable. d. Decoupling capacitor Being the system powered from output, filtering criteria is the preferred one, therefore Cmin = 2 π ⋅ f SW ⋅ RCC = 643nF min www.irf.com AN-1087 27 from which the next standardized value is chosen C = 660nF e. MOT resistor From the given relationship RMOT = 2.5 ⋅1010 t MOT = 30kΩ Layout guidelines and examples ¾ IC placement Due to the nature of the control, based on fast and accurate voltage sensing, it is mandatory to layout the circuit in order to keep the IR1167 as closest as possible to the SR MOSFET. As a general guideline, the physical distance between the two devices should never exceed 10mm (0.4 inches). ¾ IC Decoupling Capacitor The key element to proper bypassing of the IC is the physical location of the bypass capacitor and its connections to the power terminals of the control IC. In order for the capacitor to provide adequate filtering, it must be located as close as physically possible to the VCC and COM pins and connected thru the shortest available path. ¾ Differential Sensing for VD/VS IR1167S provides accurate differential voltage sensing across synchronous MOSFET. It is recommended to minimize the traces lengths and to keep it separated from the power ground as much as possible. In case of through hole devices, the device pin would be the best place where VS and GND traces connect. If SO8 MOSFET is used, traces should be kept separated until the copper area where source pins are soldered. For low side configurations, when a sensing resistor is used for current feedback in the rectifier power loop, it is highly recommended not to include it in the driving and sensing loops. This will cause some noise on the VCC, which will be properly filtered by the decoupling capacitor and the RCC series resistor, as shown in the Figure 26 schematic. www.irf.com AN-1087 28 Rcc C Rg Vgate Vcc GND OVT Vs MOT Vd EN IR1167 + OUTPUT Rmot Rsense Figure 26: output current sense resistor placement (if present). ¾ Gate Drive Loop Minimal gate drive loop will reduce requirements for damping, enhancing system robustness. Gate loop inductance plays a major role in damping requirements as shown in Figure 21. Once layout is finalized, then a “rule of thumb” estimation consists in measuring the physical loop trace length in assuming each millimeter (1mm = 39.37mils) accounts for 1nH. Other methods include measurement (low frequency RCL meters or current slope for a given voltage pulse) or FEM simulations. ¾ Single layer board layout examples to supply Figure 27: Single side board, TO220 MOSFET, OVT grounded, solder side view, through hole gate resistor. www.irf.com AN-1087 29 Figure 28: Single side board, TO220 MOSFET, OVT grounded, solder side view, SMD gate resistor. Figure 29: Single side board, TO220 MOSFET, OVT to VCC, solder side view, through hole gate resistor. Figure 30: Single side board, SO8 MOSFET, OVT to VCC, solder side view. www.irf.com AN-1087 30 Figure 31: Single side board, SO8 MOSFET, OVT grounded, solder side view. Symbols list VTH1: IR1167 turn off threshold VTH2: IR1167 turn on threshold VTH3: IR1167 periodic logic (reset) threshold RDSon: synchronous rectifier MOSFET channel ON resistance ID: synchronous rectifier MOSFET drain current VDS: synchronous rectifier MOSFET drain to source voltage MOT: IR1167 minimum ON time parameter tblank: IR1167 turn off blanking time T1: primary switch ON time in flyback converter T2: secondary current duration in flyback converter T3: primary AND secondary circuit inactivity time in DCM flyback converter C: IR1167 decoupling capacitance value on supply Rg: SR MOSFET gate drive loop resistance external to IR1167 IC RMOT: Minimum ON Time programming resistor value RCC: supply voltage series resistor value (Vsupply to VCC) fSWmax: converter maximum operating switching frequency fSWmin: converter minimum operating switching frequency TICamb: ambient temperature with reference to IC (most cases is PCB temperature where IC is soldered) Vsupply: available voltage source for IC power up OVT: Offset Voltage Trimming of turn on and off comparator Qg: SR MOSFET total gate charge Qgd: SR MOSFET gate to drain (Miller) charge www.irf.com AN-1087 31 Vgs: SR MOSFET gate to source voltage Vg_high: IR1167 gate drive output voltage IQCC: IR1167 quiescent current Rg loop: total gate loop resistance Lg: total gate loop parasitic inductance Ciss: SR MOSFET input capacitance Eg: SR MOSFET stored in gate Pdr: total power dissipated by the gate drive function RSource: gate driver source resistance RSink: gate driver sink resistance PRg: gate resistance dissipated power PICmax: IR1167 IC maximum power dissipation TIC_amb: IC environment temperature (PCB temperature) RθJA: IR1167 IC junction to ambient thermal resistance VCC: supply voltage on IR1167 pin ICC: supply current on IR1167 pin UVLO: IR1167 Under Voltage Lock Out Vsupply: system available supply voltage for SR function Cmin: minimum calculated decoupling capacitance ∆VCC: supply peak to peak ripple voltage on IR1167 VCC pin tMOT: obtained minimum on time τgate off: gate loop turn off time constant NSEC: transformer secondary winding number of turns NPRI: transformer primary winding number of turns References [1] IR1167S Smart Rectifier control IC datasheet, International Rectifier, February 2006 [2] M.T. Zhang, M. Jovanovic, F. Lee, “Design considerations and performance evaluations of Synchronous Rectification in Flyback converters”, IEEE Transactions on Power Electronic, VOL.13, N.3, May 1998 [3] Smart Rectifier™ increases Power Density in Flyback Topologies while reducing System Complexity, Maurizio Salato, Adnaan Lokhandwala, Marco Soldano, and Helen Ding, PCIM China 2006 [4] Novel Output Rectification Control IC Improves Efficiency & Thermal Performance in External AC-DC Power Supplies, Adnaan Lokhandwala, Maurizio Salato and Marco Soldano, Portable Power Developers Conference 2006 [5] US Patent Application Publication N. 2005/0122753 A1, Jun. 9, 2005 Rev. 1.1 – March 2006 www.irf.com AN-1087 32