LINEAR AN39-1 Data Sheet

Application Note 39 February 1990

Parasitic Capacitance Effects in Step-Up Transformer Design Brian Huffman

One of the most critical components in a step-up design Figure 2 shows the high frequency current paths of the like Figure 1 is the transformer. Transformers have para- parasitic capacitors. In the analysis of operation assume sitic components that can cause them to deviate from the input and output voltages are at AC ground. Thus, the their ideal characteristics, and the parasitic capacitance parasitic capacitors are all in parallel. The transformer’s associated with the secondary can cause large resonating secondary provides the AC current path for these capaci- current spikes on the leading edge of the switch current tors. The current flowing through the secondary produces waveform. These spikes can cause the regulator to exhibit N times the current in the primary. As the parasitic ca- erratic operating conditions that manifests itself as duty pacitance and turns ratio increase, the primary current cycle instability. This effect is exacerbated in very high becomes progressively larger. voltage designs. Attention to transformer design will cure this problem.

D1 MUR1100

8V TO 15V

VIN C1

MUR110

D2 LT1070 MBR360**

VFB GND VC 1.82k*

AN39 F01

* = 1% FILM RESISTORS ** = OPTIONAL—SEE TEXT FOR DETAILS MBR360 = MOTOROLA MUR1100 = MOTOROLA L1 = COILTRONICS* CTX30203C04

Figure 1. High Voltage Power Supply

CPS CD

MUR1100

IPRI = N(IPS + IS + ID) CS

ISECONDARY

MUR1100 = MOTOROLA AN39 F02

L1 = COILTRONICS CPS = PRIMARY-TO-SECONDARY INTERWINDING CAPACITANCE CS = SECONDARY DISTRIBUTED CAPACITANCE CD = DIODE CAPACITANCE N = TURNS RATIO

Figure 2. AC Current Paths for Parasitic Capacitors

an39f

AN39-1