AN3372 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 37
Technical content
Datasheet sections
- 1 General circuit description
- 2 Lamp power calculation
- 3 STEVAL-ILH004V1: electrical scheme
- 4 STEVAL-ILH004V1: layout
- 5 STEVAL-ILH004V1: BOM list
- 6 PFC design rules
- 6.1 Specifications
- 6.2 Operating conditions
- 6.3 Power components
- 6.3.1 Input capacitor
- 6.3.2 Output capacitor
- 6.3.3 Boost inductor
- 6.3.4 Power MOSFET selection
- 6.3.5 Boost diode selection
- 7 Auxiliary power supply
- 8 ST7 microcontroller application pin use
- 9 Half bridge operation
- 10 Half bridge design criteria
- 11 Lamp operating point
- 12 STEVAL-ILH004V1: experimental results
- 12.1 Half bridge section
- 12.2 PFC section
- 12.3 Conducted emission pre-compliance tests
drives the lamp in low frequency square wave. L6382D5, activating all the power switches. Figure 1. STEVAL-ILH004V1 image
1 General circuit description
- The boost converter which regulates the DC bus voltage and corrects the power factor.
- The inverter stage composed of a full bridge that converts the DC current coming from the PFC stage into an AC current for the lamp. Operation mode of the full bridge realizes a buck converter. The full bridge, moreover, supplies the igniter block to generate the high voltage pulses.
Figure 2. 150 W HID ballast block diagram
- VL = inductor voltage
- Vdc = DC bus voltage
- Vlamp = lamp voltage. The duty cycle D is established by a current mode control circuit (see Figure 3). !-V 6HQVLQJFLUFXLW $&PDLQV ,JQLWHU 67/,7( .=&' 3)& )LOWHU %ULGJH $X[LOLDU\\9ROWDJH 9LSHU/1 =&' %ULGJH /DPS 9ROWDJH &XUUHQW VHQVH 67)101'677+/ 67)101' /,&+ 67)101' lampdcL VVV −=
AN3372 Lamp power calculation Doc ID 018625 Rev 1 7/37
2 Lamp power calculation
The lamp power is obtained multiplying the lamp voltage signal for the lamp current. The lamp voltage is sensed directly across the lamp and the lamp current is obtained by the relations reported below. Starting from peak inductor and considering that the half bridge works in transition mode, the average value is: Equation 3 where:
- Ilamp = lamp current
- IAV = inductor average current
- Ipeak = inductor peak current
- ΔI = inductor current ripple. The inductor current is sensed with a current transformer and the output signal is proportional to the average value. Considering that the inductor current is equal to lamp current, and the lamp voltage is directly measured by the MCU, in this way it is possible to calculate the lamp power simply multiplying the current for the lamp voltage: Equation 4 This relation is valid because the average current is equal to the lamp current. This formula is implemented in ST7 microcontrollers in order to calculate the lamp power. I I II peak AVlamp Δ=== lamplamplamp IVI ×=
3 STEVAL-ILH004V1: electrical scheme
Figure 5. PFC electric scheme
Figure 6. Half bridge electric scheme
Figure 7. Auxiliary power supply electric scheme and microcontroller pin use
4 STEVAL-ILH004V1: layout
Figure 8. Bottom view (not to scale) Figure 9. Top view (not to scale)
5 STEVAL-ILH004V1: BOM list
Table 1. BOM
Table 1. BOM (continued)
3.2 Apk
2.6 Apk
6 PFC design rules
The front-end stage of conventional offline converters, typically consisting of a full-wave rectifier bridge with a capacitor filter, has an unregulated DC bus from the AC mains. The filter capacitor must be large enough to have a relatively low ripple superimposed on the DC level. The current from the mains is a series of narrow pulses with very high amplitude. A consequence of this condition is the distortion of the AC line voltage and a poor utilization of the power system's energy capability. This can be measured considering two parameters: 1. the total harmonic distortion (THD) 2. the power factor (PF) Two methods of controlling a PFC preregulator are currently widely used: 1. the fixed frequency average current mode PWM (FF PWM) 2. the transition mode (TM) PWM (fixed ON-time, variable frequency) In this application the PFC section is realized with a boost converter working in transition mode and digitally controlled. Design criteria of the PFC stage power components are explained.
6.1 Specifications
- Minimum mains voltage (RMS value): Vacmin = 85 V
- Maximum mains voltage (RMS value): Vacmin = 265 V
- Minimum main frequency: fmin = 47 Hz
- Expected bridge efficiency: ηbridge = 95%
- Rated lamp power: Plamp = 70 W
- Rated out power:
- Output current:
- Regulated DC output voltage (DC value): Vout = 410 V
- Maximum output low-frequency Vout ripple: Voutx = 20 V
- PFC minimum switching frequency: fmin = 30 kHz
- Expected PFC efficiency: ηPFC 96%
- Expected Input section efficiency: ηin 99%
- Expected power factor: 0.99.
6.2 Operating conditions
- Expected input power:
- Maximum RMS input current: Pout Plamp ηbridge A18.0V PI out out out == W5.7799.096.095.0 150PP inPFCBridge lamp in =⋅⋅=η⋅η⋅η= A92.0 99.085 5.77 PFV PI minac in in =
- Maximum peak inductor current:
- Maximum RMS inductor current:
- Maximum RMS diode current:
6.3 Power components
6.3.1 Input capacitor
To calculate the input capacitor the following relationship can be used: Equation 5 Using r=0.25 a commercial value of 220 nF was selected. A bigger capacitor improves the EMI behavior but worsens the THD.
6.3.2 Output capacitor
The output bulk capacitor selection depends on the DC output voltage and the converter output power. Equation 6 Considering the tolerance of the electrolytic capacitors and in the capacitor divider two capacitors are connected in series, 100 µF 250 V was selected.
6.3.3 Boost inductor
The boost inductor must be calculated at minimum and maximum Vac. The minimum inductance value must be selected. Equation 7 Equation 8 Equation 9 A61.292.022I22I inLPK =⋅⋅=⋅⋅= A06.192.0 2I in1L =⋅=⋅= A53.0 V V 24II out minac pk1L10D =⋅ nF220851.0k302 92.0 Vrf2 IC minacminsw in F3120410474 VVf4 PC outoutmin out outminsw acout ac VPinf2 )V2V(V1L ⋅⋅⋅ ⋅−⋅= mH1.1= ⋅⋅⋅ ⋅−⋅= 42077.530k2 85)2(420185L 1max mH3.1=⋅⋅⋅ ⋅−⋅= 42016630k2 265)2(420265L min 1
For this application boost inductance of 1 mH has been chosen.
6.3.4 Power MOSFET selection
For Power MOSFET selection the breakdown voltage and the RDS(on) must be considered. The MOSFET used in this section is the STF11NM60ND. Thermal measurements have confirmed this as the right choice for this device.
6.3.5 Boost diode selection
The PFC section is realized with a boost converter working in transition mode. The STTHxL06 family, which is using ST Turbo2 600 V technology, is specially suited as the boost diode in discontinuous or transition mode power factor corrections. The selection criteria is based on breakdown voltage and current. A rough selection can be performed adopting the following criterion:
- The breakdown voltage must be higher than (Vout + ΔVop) +margin
- The diode current must be higher than 3 times the average current Iout. In this case STTH1L06 has been chosen. The average diode current is: Equation 10 Use the following equation to evaluate the conduction losses: Equation 11 Considering Tjmax = 150 °C and the maximum ambient temperature Tambmax = 50 °C, it is possible to calculate the RTHJ-amb as follows: Equation 12 The calculated Rth is higher than the STTH1L06 (diode RTj-a =70 °C/W) thermal resistance junction-ambient, so no heatsink is needed. In any case, thermal measurement confirms the real device temperature. A18.0V PI lamp out out == W22.0=⋅+⋅= 2 D10outD10 I0.165I0.89P W/C465°=−=−= 0.43 50150 P TTR diode ambm axjmax amb-THJ
7 Auxiliary power supply
resistor divider is designed in order to set 3.3 V on the FB pin when output voltage is 15 V. Figure 10. Auxiliary power supply
8 ST7 microcontroller application pin use
Figure 11. ST7FLITE49K2 pinout
- Pin 1: not used
- Pin 2: not used
- Pin 3: MCU PWM3. Output used for PFC gate driver
- Pin 4: not used
- Pin 5: not used
- Pin 6: MCU reset: In Figure 12 the reset circuit is shown. !-V 633! 6$$! !).0" #,+).!).0" !).0" 633 /3##,+). /3# 0"!).3#+ 0"!).-)3/ 0"!).-/3) 0##/-0).! 0##/-0)." "2%!+ 0#,4)##/-0)." 0#)###,+ 0!(3 !407- 0!(3 !4)# 0#"2%!+ EIX ASSOCIATED EXTERNAL INTERRUPT VECTOR (3 M! HIGH SINK CAPABILITY )##,+0!(3 2%3%4 !407--#/0!(3 6$$ !407-0!(3 !407-0!(3 EI EI EI
In Figure 14 the circuit for Vlamp measurement is shown.
- Pin 17: MCU analogue input PB3 (AIN3) used for lamp current measurement.
Figure 15. Isense measurement circuit
- Pin 18: MCU analogue input PB4 (AIN4). This pin is used for Vin input mains measurement.
- Pin 19: not used
- Pin 20: not used
- Pin 21: not used
- Pin 22: MCU PC0. Zero current detect for half bridge current. !-V Q)9 700%$7 700%$7 ,6(16
Figure 19. PFC OC circuitry
- Pin 29: general purpose input-output pin. This pin is used to switch on a green LED indicating that all the electrical parameters are in the right range and the board is working.
- Pin 30: low side input MCU PWM1. Output used to drive half bridge low-side gate driver.
- Pin 31: CSO general purpose input-output pin. This pin is used to switch off all the circuit if the voltage on this pin is high.
- Pin 32: general purpose input-output pin. This pin is used to generate low frequency signal for half bridge. !-V 3)&B2&
AN3372 Half bridge operation Doc ID 018625 Rev 1 27/37
9 Half bridge operation
After the startup of the auxiliary power supply gives the right voltage at the L6382D5, the auxiliary supply voltage for the microcontroller is generated. After this sequence the microcontroller gives the right startup signals at the driver. Ignition phase The ignition voltage is generated by the high voltage transformer. On primary winding a voltage is generated by means of a capacitive discharge. The differential of potential is transferred (high voltage) at secondary winding causing lamp ignition. The voltage level across the lamp is about 3 kV. Warm-up phase During this phase a relative high current must be supplied to avoid the lamp extinguishing. In this case the maximum lamp current is limited at 30% higher than the nominal value. Burn phase In this phase the lamp power must be controlled regulating the lamp current. To avoid acoustic resonance the lamp is driven with low frequency square wave current lamp current. Current sensing The current is monitored measuring the voltage across an Rsense obtained by a current transformer. This signal is filtered in order to obtain information on the real average current that flows in the lamp. This signal is the feedback used by the MCU. Voltage sensing The lamp voltage is obtained measuring the voltage across the lamp, filtering this signal, and sending this signal to ADC of the microcontroller. In this way the lamp voltage is controlled directly by the MCU. Lamp power management The lamp power is obtained multiplying the lamp voltage signal for the lamp current. The lamp voltage is sensed directly across the lamp. The lamp current is obtained considering the feedback signal coming from the current sense circuit. The MCU calculates the lamp power multiplying the value of lamp current for the lamp voltage.
Half bridge design criteria AN3372 28/37 Doc ID 018625 Rev 1
10 Half bridge design criteria
The design of the half bridge section involves the magnetic component and the device selection. Buck inductor Consider that, in this project, an inductor of 600 µH was selected. PMOS selection The selected PMOS for half bridge is: STF11NM60ND. The system shows very good efficiency using this device.
AN3372 Lamp operating point Doc ID 018625 Rev 1 29/37
11 Lamp operating point
For correct operation in each phase different electrical values (voltage and current) must be applied. The chosen values for each phase are reported below. Ignition phase The ignition voltage, in case of a cold lamp, is about 3-5 kV. The ignition voltage increases with increasing lamp temperature. The ignition voltage in case of a hot re-strike can reach 25 kV . The circuit is not designed for hot re-strike. Warm-up phase During this phase a high warm-up current must be supplied (30% higher than nominal current) to prevent the lamp extinguishing. The lamp voltage increases gradually starting from a quarter to nominal lamp voltage up to the nominal value. The warm-up time is about 2 minutes. For the specified lamp, 70 W metal halide lamp, a current of 1 Arms was applied. Burn phase The lamp is designed to be driven with a low frequency square wave AC current to avoid acoustic resonance of the electric arc. Acoustic resonance occurs approximately in the frequency domain: 1 kHz - 1 MHz. Some frequency range free of acoustic resonance exists. The commutating frequency of the half bridge should be limited to the domain 50 Hz - 1 kHz to avoid any risk on acoustic resonance. In this application the commutating frequency has been chosen in 160 Hz. The nominal lamp voltage during burn phase is approximately 100 V and the nominal lamp power is 70 W. The differential resistance of the lamp is small and negative. To obtain a stable operating point, inductive impedance in series with the lamp is needed.
12 STEVAL-ILH004V1: experimental results
12.1 Half bridge section
current and the lamp power are maintained constant, some waveforms are shown below. Figure 20. Steady-state phase: lamp current, voltage and lamp power
Figure 21. Steady-state phase: inductor current and lamp voltage
12.2 PFC section
Figure 22. STEVAL-ILH004V1: power factor vs. input voltage
12.3 Conducted emission pre-compliance tests
that emission levels are below the limits. Figure 25. STEVAL-ILH004V1: peak measurement
13 Conclusion
Several tests on HID ballast have been performed. Using the proposed setting the ballast works properly according to all the lamp specifications. The system shows very good efficiency.
14 Reference
- AN2747 application note 2. L6382D5 datasheet 3. ST7LITE49K2
Table 2. Document revision history 09-Jan-2013 1 Initial release.