CONFIDENTIAL
1 of 29
Doc. AN-100768 V1.0
WWW.CISSOID.COM
The Leader in High Temperature
Semiconductor Solutions
CHT-STROMBOLI
Application Note 15-40_S12-25
Version: 1.0
03-Dec-2010 (Modified)
Table of contents
I Introduction ........................................................................................................................ 2
II Block diagram .................................................................................................................... 3
III Detailed Schematics (Figure 2) ......................................................................................... 4
IV Understanding the CHT-STROMBOLI .............................................................................. 7
IV.1 Transformer design .................................................................................................... 7
1. Turn-ratio ....................................................................................................................... 7
2. Primary inductance ........................................................................................................ 7
3. Choice of the transformer core ...................................................................................... 8
IV.2 Input capacitor ......................................................................................................... 12
V The preregulator .............................................................................................................. 14
VI The controller .................................................................................................................. 15
VI.1 Power-good flag ....................................................................................................... 15
VI.2 Switching frequency clock ....................................................................................... 16
VI.3 Current sensing transformer and comparator ......................................................... 17
VI.4 Ramp compensation ................................................................................................ 18
VI.5 Voltage loop ............................................................................................................. 19
VI.6 Output voltage drop dependency vs. Iout ................................................................ 19
VI.7 Current limiting ......................................................................................................... 20
VI.8 Caution on current limiting operation ....................................................................... 20
VI.9 Loop transfer function .............................................................................................. 20
VI.10 Soft start ............................................................................................................... 20
VII Feedback converter ......................................................................................................... 22
VIII Output rectification .......................................................................................................... 24
VIII.1 Current-driven synchronous rectifier ................................................................... 24
VIII.2 Output capacitor .................................................................................................. 27
IX List of figures ................................................................................................................... 28
XI Contact & Ordering .......................................................................................................... 29
XII Disclaimer ........................................................................................................................ 29
3-Dec-10 Contact : Gonzalo Picún (+32-10-489214)Dec. 10
ApplicationNote 15-40_S12-25
(Modified)
CONFIDENTIAL
Doc. AN-100768 V1.0
WWW.CISSOID.COM
2 of 29
I Introduction
This document describes in details the implementation of a constant frequency current mode
flyback isolated DC/DC converter CHT-STROMBOLI based on CISSOID’s high-temperature
devices CHT-MAGMA, CHT-NMOS8010, CHT-NMOS4020 and CHT-SNMOS80. Small and
medium signal diodes are created by using the bulk diode of high temperature MOSFET:
CHT-SNMOS80 with gate and source shorted.
All CISSOID components are rated from -55°C to +225°C (Junction temperature. The CHT-
STROMBOLI reference design is specified -55 to 175°C with brief excursions to 225°C. The
passive components are rated at +150°C but will permit brief excursions above this value,
with a reduced reliability.
The CHT-STROMBOLI can be operated from 15V up to 40V and the implemented topology
ensures a cycle-by-cycle current limitation, making the converter very robust to every fault
condition present at the output.
A voltage input comparator (Under Voltage Lockout) switches off the converter when oper-
ated below the minimum specified voltage. This provides a small input power and a reliable
soft start under these conditions.
Two symmetric outputs are provided and an accurate voltage feedback guarantees a high
stability over the load or the input voltage variations, making the design very robust to all kind
of external perturbations.
The detailed performances of the CHT-STROMBOLI can be found in the related product
datasheet. The global specifications are as follows:
Input voltage range: 15 to 40V
Output voltage: symmetrical +/-12V
Output power: 25W
Efficiency>70% at maximum power, over the whole temperature range
150kHz center frequency
In the following sections it is assumed that the general theory of flyback current mode con-
verters is known. The passive elements in each block will be described.
Figure 1 shows a generic block diagram of CHT-STROMBOLI, along on the next pages the
detailed schematics of the reference design, in figure 2. The next chapters of this document
will describe in details the different functionalities.