EVAL6520-1421 Datasheet, PDF (5/16 Pages) STMicroelectronics – Standard form factor

EVAL6520-1421
Board description
It is now possible to estimate the ignition frequency, which must be higher than 46 kHz
(minimum programmable value), and the minimum preheating frequency that guarantees a
preheating voltage higher than 130 Vrms, as required by the lamp specifications.
Equation 3
V_PFC ⋅
π
V_PH−IGN =
1
21
⋅ _jωCRES
jωC_RES ^{+ jωLRES}
A preheating frequency equal to 70 kHz has been selected by connecting a 2.49 kΩ resistor
(R5) between the FPRE pin and GND.
The half bridge is based on two STT13005D power bipolar transistors (Q1 and Q2). Both the
high side and low side transistor are driven by a pulse transformer (T2).
To design this pulse transformer, the following parameters must be taken into account:
● Maximum available spacing on the PCB: this determines the core dimension.
● Maximum magnetizing current (I_mag,rms) on the primary side of the transformer: this
current causes core losses to not be transferred as a useful signal on the secondary
side of the transformer. To minimize it, a higher primary inductance should be adopted.
Typical inductances are between 6 mH and 40 mH, depending on the core dimension
and the core permeability.
● Primary to secondary transfer ratio (n): the output voltage of a step-down transformer is
lower than the input voltage, whereas the output current is higher than the input
current. This helps to obtain higher DC currents with lower IC power dissipation. The
minimum Vbe(sat) must be guaranteed in any condition as well as the minimum IB that
guarantees the saturation condition of the BJT.
The I_mag,rms(MAX) is selected lower than 10 mA when Vcc is equal to 13 V and a typical
storage time of 1.2 µs is considered, therefore the primary inductance must be:
Equation 4
Lpri
≥
Vcc ⋅
T_on,max
I_mag,rms
=
3
1
3 ⋅ Vcc ^{2 ⋅ frun − Tdt − Tsto − 200ns} = 1.732 ⋅ Vcc ^{9.42us - Tsto} = 6.18mH
I_mag,rms
I_mag,rms
The Vbe of the bipolar transistor can be calculated as follows (see Figure 2):
Equation 5
Vbe
=
Vpri
⋅
n
−
Ib
⋅
Rb
=
⎡
⎢⎣
Vcc
−
Ib
n
⋅
(Rds, on_h
+
Rpri
+
Rds, on_l)_{⎥⎦⎤}
⋅
n
−
Ib
⋅
Rb
The Rds,on_h and the Rds,on_l are the ON resistances of the L6520 drivers and can be
considered equal to 10 Ω each.
Ib is equal to Ic/hfe, and can be considered equal to Ic/6. Ib times Rb can be set between
0.7 V and 1 V, during run mode: in this design Rb (R6 and R8) can be selected between 10
Ω and 13 Ω.
Doc ID 018537 Rev 2
5/16