Catch here
only a few components, is shown in Figure 3. This converter accepts an extremely wide-ranging input—30 V to 400 V—and produces ±12 V/30 mA outputs. This circuit can achieve efficiency as high as 87% at full load for both outputs with a 30 V input. In this topology, LT8315’s GND pad is intentionally ungrounded and connected as the common switch node for driving both outputs. For PCB layout, LT8315’s GND pad’s size should be constrained within the exposed pad area to reduce electromagnetic interference to other components because the GND trace is a relatively noisy switch node in this topology. Diode D2 and two 1% resistors at the FB pin form the feedback path regulating the positive output voltage. D2 is necessary to prevent the FB pin discharging whenever the MOSFET conducts. The resistive voltage divider does not need to take into account the forward voltage drop of D2 because the forward voltage of D2 and D3 are equal and cancel; therefore, the feedback network tracks and closely regulates the positive output voltage.
The negative rail comprises a low voltage coupling capacitor CFLY, a second inductor L2, a catch diode D4, and the negative output capacitor CO2. According to the inductor volt-second balance for the circuit loop of CO1-L1- CFLY-L2, the average voltage across L1 and L2 is zero, so the coupling capacitor CFLY’s voltage is equal to the positive output voltage. CFLY charges up L2 during the on-time of the MOSFET, while D4 provides a path for the L2 discharge during the MOSFET off-time. The negative output voltage is indirectly regulated based on the voltage of CFLY remaining constant and equal to the positive output voltage. As shown in the regulation curve of Figure 4, the negative supply maintains ±5% regulation for a load range of 3 mA to 30 mA at various input voltages, when the positive load is at a full 30 mA.
This article presents two dual polarity converter solutions for a wide 30 V to 400 V input range: one isolated, the other nonisolated. The LT8315 is used in both,
EDAC hph p25_CIE 210x130 ad template 14/02/2020 10:15 Page 1
Industrial
Figure 4. Negative 12 V load regulation curves at various input voltages for the dual inductor buck converter in Figure 3
due to its high voltage integrated MOSFET, no optocoupler feedback loop, and internal high voltage startup circuit. Other features include low ripple Burst Mode® operation, soft start, programmable current limit, undervoltage lockout, temperature
compensation, and low quiescent current. LT8315’s high level of integration simplifies the design of high voltage input and dual polarity output circuits for a wide variety of applications.
analog.com
www.cieonline.co.uk
Components in Electronics
September 2020 25
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54
